Young men who have sex with men (YMSM) account for two-thirds of new HIV infections in young people in the United States. Identifying between-person and within-person correlates of sexual risk-taking provides critical information for developing behavioral prevention efforts for this group. Possible predictors of sexual-risk behavior in YMSM include major depressive disorder (MDD), post-traumatic stress disorder (PTSD), and variation in psychological distress over time. To date, research has been equivocal with regard to the relationship between psychiatric disorders, psychological distress, and sexual risk behaviors. Participants included 119 16-20-year-old YMSM. Ethnicity/race of the participants included: black/African-American (46.2%), white (19.3%), Latino/Hispanic (12.6%), multiracial (11.8%), Asian/Pacific Islander (2.5%), and other (5.9%). Sexual risk outcomes included total number of male partners and unprotected anal sex acts across four waves of data collection (24 months). The study found that the between-person correlates, including ethnicity and age, predicted total male partners. Between-person correlates, including ethnicity, MDD, and a moderating effect of PTSD on psychological distress emerged as determinants of unprotected anal sex acts.