A nationally representative study of the sexual behavior of men aged 20-39 in the United States shows that the prevalence and frequency of sexual acts (vaginal, anal and oral) and sexual orientation vary by social and demographic characteristics. Analysis of data from 3,321 respondents to the 1991 National Survey of Men reveals that 95% of men have had vaginal intercourse; among them, 23% have had 20 or more vaginal sex partners in their lifetime. About one-fifth of never-married and formerly married men had four or more partners over a recent 18-month period. However, 41% of never-married men and 32% of formerly married men did not have coitus during the four weeks preceding the interview. Only 20% of men have ever engaged in anal intercourse. Among these, 51% had not done so during the previous 18 months, and 90% had not done so during the previous four weeks. Seventy-five percent of men have performed oral sex and 79% have received oral sex, although 53% of men who ever performed oral sex had not done so during the four weeks prior to interview, and only 11% had done so six or more times. The frequency of receiving oral sex is similar. Only 2% of sexually active men aged 20-39 have had any same-gender sexual activity during the last 10 years, and only 1% reported being exclusively homosexual during this interval.