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To examine the share of steady versus casual partners as the source of HIV infection in gay male seroconversions between 1984 and 2000 and the effect of age at seroconversion on the source of HIV transmission.The sample consisted of 144 seroconverstors from the Amsterdam Cohort Study among Homosexual Men. Questionnaires and post-seroconversion interviews were used to determine the source of HIV transmission.Analysis revealed an interaction effect between calendar year and age at seroconversion (P < 0.05). Younger seroconverters had higher odds ratios [odds ratio, 11.33; 95% confidence interval, 1.77–72.13] to be infected by their steady partner late in the AIDS epidemic: 15% (three of 20) between 1984 and 1987 versus 67% (six of nine) between 1994 and 2000. No such time effect was present for older seroconverters who were consistently more likely to be infected by a casual partner: 79% (37 of 47) between 1984 and 1987, and 83% (15 of 18) between 1994 and 2000.Young gay seroconverters today are more likely to have contracted HIV from a steady partner than from a casual partner, compared with early in the AIDS epidemic and compared also with older gay seroconverters. There is a pressing need for preventive measures addressing sexual risk behaviour within steady relationships among younger gay men.