One hundred thirty-four homosexual men from a prospective cohort study of AIDS risk from New York City and Washington D.C. and 139 homosexual men from a similar cohort in Copenhagen and Aarhus, Denmark were questioned regarding their sexual practices and knowledge of their human immunodeficiency virus (HIV) status over the previous 12 months. Seventy percent of Danish men and 63% of U.S. men participated in anal intercourse during the previous 12 months. Knowledge of one's own HIV status by itself did not have any significant effect on participation in anal intercourse, partner number, or condom use. Only 23% of U.S. men and 24% of Danish men always asked potential partners about their HIV status. However, men who did ask were very unlikely to choose a partner of opposite HIV status (p< 0.006). Danish men were more likely to practice anal intercourse without a condom than were the U.S. men (p< 0.0001); however, Danes were more likely to be in a concordant monogamous relationship than were the U.S. men (p< 0.001). Fourteen percent of U.S. men and 21% of Danish men were not aware of their own HIV status and 52% of the U.S. cohort and 31% of the Danes had anal intercourse with a man whose status was unknown to them. Overall, only 32% of American and 53% of Danish homosexual men were practicing completely safe sex. We suggest that education to promote the need for awareness of one's own and one's partner's HIV status should be stressed.