Spot the difference: Doctors benefit from double vision

They came two by two. And they just kept coming. So many were there that passers-by on nearby Westminster Bridge might have suspected the sudden onset of double vision.

In fact, although no eye tests were necessary, the presence of 500 sets of female twins enjoying a party in their honour at St Thomas's Hospital yesterday might ultimately help the visually impaired, as well as those suffering from a range of other medical conditions.

The hospital's Twin Research Unit, set up in 1992, has attempted to explain the role of genetics in many common diseases of ageing - notably osteoporosis and osteoarthritis - by examining the DNA of people born from the same egg.

Wise to the commercial possibilities of the project, which could help thousands, a private firm, Gemini Research, has already backed it to the tune of pounds 1m. So successful have been the initial results that 5,000 pairs of twins are to be recruited for the next phase.

Gemini, and Dr Tim Spector, who leads the research, hope the data collected will enable doctors to predict which patients are most susceptible to diabetes, asthma, cardiovascular disease, back pain, skin ailments, deafness and obesity, as well as bone disorders. A "pill in a bottle" cure may be many years away, says Dr Spector, but his findings have already helped establish a link between osteoporosis - a disease which costs the NHS pounds 750m a year - and an individual's ability to absorb vitamin D.

While this aspect of the project has clearly paid dividends, Dr Spector admits no new light has been shed on the phenomenon of twinning itself. "We still don't know precisely why this occurs. But we do know that twins are more likely to give birth to twins."

When asked whether they had learnt anything about themselves through the testing programme, new recruit Susie Gibbon and her non-identical twin, Hillary Clewer (both 26, identically dressed and apparently able to read each other's thoughts) replied in unison: "No - but we had a lot of fun."

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