This study assessed whether sexual norms and attitudes expressed during early adolescence by minority youth from economically disadvantaged urban areas produce a sustained influence on the timing of sexual initiation. African American and Latino youth attending three middle schools were enrolled in the Reach for Health study in seventh grade and followed from an average age of 12.2 to 16.1 years. Some 849 respondents answered the question, "Have you ever had sexual intercourse" at four time points: fall seventh, spring seventh, spring eighth, and spring 10th grade. Culturally tailored scales assessed sex norms and outcome expectancies, sexual responsibility, and refusal attitudes at fall seventh grade. Influence of these norms and attitudes in early adolescence on timing of first reported sexual intercourse was examined using ANOVA controlling for gender. At fall seventh grade, 30.7% of boys and 7.7% of girls reported sexual intercourse; by spring 10th grade, the figures were 74.8% and 56.4%, respectively. Those reporting greater peer involvement in sex and more positive sex outcome expectancies were more likely to have initiated sex by fall seventh grade. Through 10th grade, the higher the scores on peer norms (f = 41.08, p < .0001) and outcome expectancies (f = 5.87, p = .002) at entry into seventh grade, the earlier the timing of initiation. Higher scores on sex responsibility at baseline were associated with delayed sexual intercourse (f = 7.36, p < .001), as are refusal attitudes (f = 15.62, p < .0001). Despite significant gender differences in timing of initiation and mean scale scores, these relationships were similar for males and females. Findings suggest the importance of addressing sexual norms and attitudes of minority youth in interventions to delay early sexual initiation in urban environments where this risk is high. Given their sustained influence on timing of sexual initiation, such interventions must begin prior to middle school and continue through mid-adolescence, years when early sexual experience can lead to negative health and social outcomes.