Trends in sexual behavior associated with incident infection with human immunodeficiency virus (HIV) type 1 are described and a case-control study was conducted to examine risk factors for HIV seroconversion in homosexual men who became infected with HIV between 1982 and 1994 from four geographic sites: Amsterdam, the Netherlands; San Francisco, California; Vancouver, Canada; and Sydney, Australia. Changes in sexual behaviors were evaluated from cohort visits in the preseroconversion, seroconversion, and postseroconversion intervals and were further examined over three time periods: 1982-1984, 1985-1987, and 1988-1994. In a case-control study, sexual behaviors, substance use, and presence of sexually transmitted disease were compared between 345 HIV-positive cases and 345 seronegative controls matched by visit date and site. Receptive anal intercourse was the sexual behavior most highly associated with seroconversion. The odds ratio (OR) per receptive anal intercourse partner increase was 1.05 (95% confidence interval (CI) 1.02-1.09). To more carefully examine risk associated with receptive oral intercourse, analyses were done in a subgroup of men who reported no or one receptive anal intercourse partner. The risk (OR) associated with receptive oral intercourse partner increase was 1.05 (95% CI 1.0-1.11). In multivariate conditional logistic regression analyses, presence of sexually transmitted disease (OR = 3.39, 95% CI 1.95-5.91) and amphetamine use (OR = 2.55, 95% CI 1.26-5.15) were independently associated with seroconversion. Although the prevalence of major risk factors has decreased over time, the associations of these behaviors and HIV infection persist, suggesting that these risk behaviors remain important avenues for public health interventions.