A nationwide study of sexual behavior was undertaken among 4,680 randomly selected Danes aged 18-59 years. The median number of sexual partners (lifetime) was highest for men aged 30-34 years (eight partners) and for women aged 25-29 years (seven partners). After adjustment for age and sex, having had greater-than-or-equal-to 5 sexual partners in the past year was strongly associated with living in larger cities, intravenous drug use, and having sex with a prostitute, a bisexual man, an intravenous drug user, or a resident of sub-Saharan Africa. The frequency of ever having had anal intercourse was highest among women aged 20-34 years (range, 27%-36%) and was independently associated only with increasing number of sexual partners (lifetime). Overall, 2.7% of men reported any homosexual experience, among whom most (88%) had also had heterosexual intercourse. Prostitute contact (ever) was reported by 13% of all men and was associated with a high educational level, a history of travel, a greater number of sexual partners, and intravenous drug use. Overall, sexual contact with someone considered at high risk of human immunodeficiency virus (HIV) infection (a homosexual/bisexual man, intravenous drug user, prostitute, or sub-Saharan African resident) was reported by 15.9% of men and 4.8% of women. Among active blood donors (past year), 12.5% of men and 4.0% of women had engaged in potentially risky behavior. HIV testing was deliberately sought more often by respondents exposed to someone at increased risk of HIV infection (10.6%) than by those unexposed (6.5%, p < 0.01). Exposure to persons at risk of HIV infection is considerable in Denmark. The majority of persons who have had potential exposure to HIV have not yet been tested for HIV.