Early sexual behaviors during adolescence have the potential to lead to unhealthy outcomes. This study explored the association between specific dimensions of maternal relationship quality and adolescent sexual debut by age 15. We hypothesized that adolescents who have poor maternal relationships are at greater risk of early sexual debut than those who have a more supportive relationship. These hypotheses were examined using both adolescents' perception of their maternal relationship quality and observed relationship quality. A sample of 268 adolescents (142 girls; 90% European American) were followed longitudinally from age 13 to age 15. The results indicate that adolescents with sexual debut by age 15 report significantly worse relationships with their mothers compared with other adolescents. Furthermore, behavioral observations of lower maternal support and higher observed displays of externalizing affect towards the mother also were related to early sexual debut. The significance of maternal relationship quality remained after controlling for family structure, race, and socioeconomic status. These findings suggest that mothers have a significant role in the timing of their adolescents' sexual debut. (Contains 6 tables.)