The two-stage parametric regression model of Brookmeyer and Goedert has been adapted and fitted to data on the development of AIDS in haemophiliacs in the UK who are seropositive for HIV. The risk of developing AIDS by a given time following seroconversion increases with increasing age at seroconversion. It is likely that the risk increases smoothly with age, although the data have been analysed in three age categories, and it is estimated that by seven years after seroconversion 6 per cent of patients aged under 25 at seroconversion, 20 per cent of those aged 25-44 and 34 per cent of those aged 45 or more have developed AIDS. For a given age at seroconversion the annual risk of developing AIDS increases with increasing time after seroconversion, and at seven years the annual risks of developing AIDS during the next year in the three age groups are estimated to be 2 per cent for those aged less than 25 at seroconversion, and 10 and 11 per cent respectively for those aged 25-44 and 45 or more.