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 ≥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. Am J Epidemiol 1992;135:593–602.