The study examined behavioral, relationship, and serostatus variables that potentially contribute to HIV infection risk in three age groups of men who have sex with men (MSM). MSM recruited in West Hollywood, California self-administered a questionnaire measuring unprotected insertive anal intercourse (UIAI) and unprotected receptive anal intercourse (URAI) with primary and nonprimary partners. The following relationship/serostatus variables were also assessed: recency of HIV testing, knowledge of own HIV serostatus, perception of partner's serostatus, seroconcordance (self and partner seronegative), and self-reported monogamy status. The prevalence of UIAI and URAI was higher with primary than nonprimary partners. These sexual risk behaviors with primary partners were substantially more prevalent among men younger than 25 years of age than among men aged 25 to 30 or over age 30. UIAI and URAI with nonprimary partners were uncommon in each age group, and there were no significant age differences on the serostatus and relationship variables. The findings suggest that young MSM may be at elevated risk for contracting HIV by virtue of their sexual risk behavior with primary partners. Targeted interventions for MSM need to address sexual risk in the context of primary relationships.