In a taxi recently, the cabbie was telling me his troubles: he was separated from his wife; one of his twin 11-year-old daughters had kidney disease and was seriously ill.

Without thinking, I said: 'Why don't you give her one of yours? Or better still, sound out her sister. She would be the best donor.'

The cabbie's face was staring at me in the mirror in disbelief - and he was angry. 'What do you mean? What do you know about it?' he said. I apologised for my thoughtlessness. Then I explained. I had donated a kidney to my brother a year before.

When compared with the possible death of a beloved child, or sister, it really was no big deal, I told him. He stopped the taxi so that I could tell him more.

Considering that I carry around a note, most of the time, stating that, in the event of my death I do not wish any of my organs to be used for transplant, it had been strange to find myself sitting up in a hospital bed in St Thomas's Hospital, in London, feeling a bit sick, after the operation.

I had expected to feel at least a little different, but we really do not need two kidneys and I have never for one moment since been aware of having only one, any more than I was aware of having two before.

Many perfectly healthy people are born with only one kidney. The one thing I could not get my head around was, as I said to my brother the evening before the operations, 'Do you realise, James, I am drinking this tea now and you will probably pee it out on Saturday evening]'

The concept of organ transplant bothers me, philosophically, as do some of the excesses of infertility treatment and genetic engineering. It smacks of too much interfering with nature; instinctively I feel the old girl probably knows what she is doing. But this was different. It was a simple case of sharing what I had.

The alternatives for my brother were to wait on a list for a transplant for someone to die with healthy kidneys, to spend the rest of his life having kidney dialysis, or to die. I had seen James at a family wedding that summer, looking pale, fatigued and obviously dependent on his drugs. He had always brought so much sunshine and laughter into our lives, with his ludicrous wit, his generosity and his gentle spirit. But he had lost his lustre and was obviously very ill. How could I possibly hesitate?

In his youth he had been a fine athlete, playing rugby and tennis, and seemed to be a healthy young man. We were all astounded, therefore, when on applying to join the Royal Navy, he was rejected on medical grounds. It was lucky that he did apply, as they discovered that he had had a kidney infection since childhood which, if untreated, would probably have killed him before he was 40. As it was, with careful monitoring, his condition could be controlled. He reapplied to the Navy and enjoyed a successful career until his mid-fifties.

Then, suddenly, the condition of his kidneys deteriorated dramatically and life became a struggle for him. Since he and I are very alike physically, I had not the slightest doubt that our tissue would match. Apart from the fact that he is male and I am female, we are virtually identical, having the same shaped hands and feet, the same colouring, and similar features. I was the obvious candidate.

When I arrived at the hospital the physician caring for James tried to talk me out of it. 'You do realise, Ms Wright,' he said, 'that any surgical procedure carries an element of risk, which it is my duty to point out to you?'

'Of course,' I replied, 'but it is not particularly high in a perfectly healthy person, is it?'

'No, indeed, and if this were surgery required for a medical condition troubling you, we probably would not place a great deal of emphasis on it. But, since it is essentially elective surgery, we must be absolutely sure that you know what you are undertaking.'

'Thank you, doctor. So, aside from that risk, what exactly am I undertaking?'

He explained that the surgery itself was more serious for me than for my brother, as they would not remove his kidneys, but simply install the new one at the side of the abdomen, not very far under the skin. James's problems, if any, would derive from the dangers of rejection, and he would have to take drugs immediately.

For me, they would have to go in much further, giving themselves plenty of room to remove the kidney without damage. I did not really want to know, but I asked: 'Where do they cut, and how big is the incision?'

I almost fled the building when he told me that the cut would be about 12 inches, coming round my waist from near the centre of my back. As for the scar, middle-age spread long ago consigned the question of whether I could wear a bikini after surgery to the realms of the academic.

The doctor estimated that I would probably stay in hospital for a week to 10 days, and then need three to four weeks of taking life gently before being completely back to normal.

I was very keen to get on with it, but in the event it was another seven months before the operation finally went ahead; I had to go back three or four times for various blood, Aids and DNA tests.

The timing was perfect. The date, would you believe, was Friday 13 November. I had moved house just one week earlier, and was tired and tense - almost regarding a few days away from home, even under these circumstances, as not unwelcome. We were admitted on Thursday afternoon, and there was very little for me to do, although James had a few more tests to undergo.

Everybody was incredibly nice to us, and I was treated like visiting royalty. The nurses, doctors, auxiliaries - everyone was helpful, kind and caring, and at the same time full of laughter.

The next morning, I was wheeled down to theatre and only had time to admire the anaesthetist's twinkling blue eyes before I was gone. The next thing I knew, I was back in bed, sitting up as described. I had a thing called a catheter emerging from my southern region, which saved me the trouble of going to the loo; it was a bit inconvenient (and very undignified) when I was ready to get up and walk around, but they removed it on the third day, quite painlessly, and that was that. I was stiff and sore but as everyone was still treating me with so much care and attention, I had no complaints. Five days later, in view of the speed of my recovery, and because my sister-in-law, James's wife, is a former nursing sister, I was discharged into her care. My GP could remove the stitches after 10 days. James had to stay in a little longer, because his care after the operation was more complicated than mine.

For me, that was all. I was not really back to full top form until after Christmas, but then who is ever on top form in that part of the year?

As for the scar, the surgeon had told me that it would be practically unnoticeable in about six months. It was longer than six months and it is not invisible, but it is certainly very neat. I probably would not think twice about wearing a bikini this year if I was built for it.

It did take a few months for the nerve endings along the scar to sort themselves out, so I had a numb feeling at some points along the line for a while.

James made an instantaneous and total recovery, but one more hurdle was required for him. There has been no rejection, and his new kidney is functioning wonderfully well, but his existing kidneys (which his wife christened Mutt and Jeff) were so badly diseased that they seriously interfered with his recovery and had to be removed. This was done, by keyhole surgery some months later, and shortly after that he began to recover fully.

Please, if you are close to anyone in this situation, do not hold back - I promise you, it is infinitely worth the temporary discomfort and minimal risk involved.

(Photograph omitted)