A growing body of research shows that gay men in primary relationships engage in higher rates of unprotected anal intercourse than single gay men. We sought to (a) describe patterns of HIV risk taking among young gay men in boyfriend relationships and (b) identify characteristics of relationships that were most associated with engaging in unprotected intercourse. The data come from the Young Men's Survey, an ongoing longitudinal survey of 416 gay men aged 18 to 27 in three medium-sized West Coast communities. Fifty-one percent of the men with boyfriends had engaged in unprotected anal intercourse in the past 2 months, compared with 21% of the single men. Forty-two percent of the respondents who had unprotected intercourse with their boyfriend did not know their own and their partners' HIV status. Factors associated with unprotected intercourse included greater relationship involvement and time spent together, sexual behavior patterns, less involvement with the gay community, poor sexual communication skills, and misperceptions about safe sex. The findings highlight the need for HIV risk reduction interventions designed specifically for young gay men with boyfriends and identify critical areas to target in such interventions.