Wimbledon v Chelsea will sell out. Travelling costs are minimal for both sets of supporters and Highbury's capacity is insufficient to cope with Chelsea's new mass support, let alone Wimbledon's steadily growing hard core.
Middlesbrough v Chesterfield is a different matter. As last year, the Manchester semi-final is likely to be played before a backdrop of empty seats: a fine advertisement for the World Cup 2006 bid.
Last year's problem was the cost of tickets. Too many were at the top end, pounds 38, not enough were at pounds 12. Fans of Liverpool, in particular, and Aston Villa, refused to pay and there were 15,000 empty seats. The FA's vapid explanation - the views were so good they had to charge premium prices - led to the novel suggestion that scaffolding should be erected in front of them to justify lower prices.
On this matter the FA has seen sense. Same ground, same views, different price structure. The FA said yesterday there would be "significantly fewer" seats at pounds 38 and many more at pounds 12 and pounds 22. In most seasons that would create a full house, even at 55,000-capacity Old Trafford.
But this year is different. For varying reasons both clubs may be relatively poorly supported. For Boro fans the match could be the filling in a Wembley sandwich. They seem likely to be in the Coca-Cola Cup final the previous Sunday (they defend a 2-0 first-leg semi-final lead at home to Stockport tomorrow) and most supporters will expect them to defeat Chesterfield. In the circumstances, many will save their cash rather than travel to Manchester. They could not even fill the away end at Derby on Saturday. Chesterfield, a relatively depressed area following the collapse of the coal industry, simply do not have many supporters.
Of course, these circumstances are not the fault of the FA, nor is Old Trafford's vast capacity. The choice of ground is valid: everyone who wants to go will be able to, the pitch is perfect (an important consideration after Villa Park's "beach" last year) and it does not add to fixture congestion - Manchester United are away that weekend.
What is under the FA's control is ticket prices. These, however, were set long ago when the usual suspects could have been expected in the semis, Manchester United, Arsenal and company. There was thus no scope for adjusting prices (which have been static for four years) to likely demand. There is not even a family concession, which could have been a good way of turning the occasion into a day out.
Final judgement will have to be reserved until the mix of ticket prices is decided. This correspondent's view could be unduly pessimistic: more than 30,000 seats will be under pounds 25 and they should sell, while the allocation of 17,000 seats at pounds 38 should be substantially reduced. The television coverage will also help, with fewer fans having access to the Old Trafford match as it is being shown on Sky.
The FA's problem is that even a 45,000 crowd leaves 10,000 spaces, and empty seats are so much more visible than gaps on terracing used to be.
FA CUP Semi-finals (13 April): Wimbledon v Chelsea (12.0) (at Highbury); Middlesbrough v Chesterfield (3.0) (at Old Trafford). Possible replays (Tuesday 22 April): Wimbledon v Chelsea (7.45) (at Highbury); Middlesbrough v Chesterfield (7.45) (at Hillsborough).