CONTEXT: Understanding how young men's sexual risk behaviors change during the transition from adolescence to early adulthood is important for the design and evaluation of effective strategies to reduce the transmission of HIV and other STDs. METHODS: Data from three waves of the National Survey of Adolescent Males (1988, 1991 and 1995) were used to categorize 1,880 respondents into clusters according to sexual risk behaviors. Univariate and bivariate analyses were conducted to assess associations between clusters and rates of self-reported STD diagnoses and positive chlamydia tests. RESULTS: Two dimensions of sexual risk-taking defined the clusters: partner characteristics and condom use. More than 50% of men remained in low-risk groups over time. In the first two waves, 24-32% of men reported engaging in high-risk behaviors (risky partners, condom nonuse); these behaviors were associated with elevated levels of STD outcomes. Nearly 40% of men who entered a high-risk group in the first two waves transitioned to a lower risk group by the third wave. Nine percent of men either engaged in increasingly risky behaviors or maintained membership in high-risk groups; elevated STD rates characterized both trajectories. Low condom use combined with having multiple partners during adolescence was associated with elevated STD rates in the year preceding the third wave; high condom use coupled with having risky partners was not. CONCLUSIONS: The prominence of low-risk behaviors over time suggests that most young men avoid sexual risk-taking. Effective strategies to reduce HIV and STD risk in young men must simultaneously address multiple dimensions of sexual behavior.