To measure the magnitude of risk reduction within a cohort of homosexual men, questionnaire responses in April 1984-March 1985 were compared to those in October 1986-September 1987. A total of 361 subjects were eligible (231 seronegative, 130 seropositive). The annual number of sex partners declined significantly from a median of 8.0 to 5.0 with no difference between the serologic groups. The number of subjects reporting no receptive anal intercourse increased as did condom use during anal receptive intercourse. More seronegatives than seropositives subjects reported no condom use during receptive anal intercourse with regular partners (45.7 per cent versus 23.4 per cent), and with casual partners (14.9 per cent versus 1.5% percent). Among subjects with the most casual sexual contact at the second visit, 33.3 per cent of seronegatives and 29.2 per cent of seropositives did not report usual condom use during receptive anal intercourse with casual partners. Although we have documented marked risk reductions, safe sex practices are still not universal, and a few individuals continue to put themselves at extremely high risk.