Lesbians, gays, and bisexuals (LGBs) are at increased risk for alcohol use during young adulthood, but the mechanisms remain inadequately understood. The aim of the present study was to examine the trajectories and determinants of alcohol use among LGB young adults who were sampled prospectively. The sample included 111 LGB individuals (47 women and 64 men) and 2,109 heterosexuals (1,279 women and 830 men), who were assessed at three time points: during the summer after their senior year of high school and during the fall and spring of their freshman year of college. Hierarchical linear modeling analyses indicated that lesbians consumed more alcohol than their heterosexual peers during high school, whereas gay men increased their alcohol use at greater rates than heterosexual men during the initial transition to college. Positive alcohol expectancies and social norms mediated this relation for both men and women. The results extend the generalizability of these processes and highlight the importance of considering normative social-cognitive influences in the development of alcohol use among LGB young adults.