CONTEXT: The physical health detriments associated with adolescent females' having older romantic partners are well documented. However, little is known about the relationship between having an older partner and females' subsequent mental health. METHODS: Two waves of data from 1,440 participants in the National Longitudinal Study of Adolescent Health were analyzed. The sample was restricted to females in grades 7-12 who had not had sex at Wave 1 (1994-1995) and reported at least one romantic relationship by Wave 2 (1996). A lagged dependent variable approach with ordinary least-squares regression measured changes in depression and self-esteem associated with sexual and nonsexual relationships with same-age and older partners. Intimate partner violence was tested as a mediator. RESULTS: Compared with respondents reporting a nonsexual relationship with a same-age partner, those reporting a nonsexual relationship with an older partner, sex with a same-age partner or sex with an older partner experienced greater increases in depression between surveys; mean predicted depression levels at Wave 2 ranged from 7.7 to 9.0 across these groups (possible range, 0-27). Intimate partner violence explained one-third of the difference between those who had had sex and those who had not had sex with same-age partners. Fewer associations were found for self-esteem, and differences between groups were small. CONCLUSIONS: Health correlates of adolescent sexual behavior go beyond physical health outcomes. Future research should identify mechanisms through which relationships, especially those with older partners, are associated with declines in mental health.