To describe the occurrence of sexual behaviour involving increased risk of HIV-infection in a representative sample of Danish men, 16-55 years of age, a self-administered anonymous questionnaire was distributed in connection with a personal health interview and returned by mail. The response rate was 55% corresponding to 1,155 participants. The sexual behaviour of 33% of the men was potentially risky, with a chance of HIV-infection. Homosexual experience was reported as 3% and sexual contact with female prostitutes as 14%. During the previous twelve months, 1% had had homosexual contact, 2% had had contact with prostitutes, and 22% had more than one female partner; most of these men had "one night stands" and used condoms infrequently. Consequently, a significant minority of the participants engaged in sexual behaviour which might facilitate the dissemination of HIV-infection. Although the prevalence of HIV among heterosexuals in Denmark is low, intense health education will be needed in order to prevent heterosexual spread of HIV in the future.