The prevalence of antibody to HTLV-III has increased from 3.7% (4/107) amongst unselected British homosexual men attending a London sexually transmitted disease (STD) clinic during one week in March, 1982, to 21% (26/124) in those attending during one week in July, 1984. Seropositive men had a significantly higher prevalence of infection with hepatitis B virus than did seronegative men. 82% (27/33) of the seropositive men in 1984 were symptomless or had only local genito-urinary symptoms referable to the STD for which they were attending. The evidence suggests that HTLV-III was initially an imported but is now an endemic sexually transmitted agent. As of July, 1984, at least 2600 homosexual men in London would probably have been infected.