Early deviance can interrupt adolescent development, such as education. Substance use in early adolescence has been studied as a risk factor for school dropout. Few studies examine the relationship with diverse samples. In this study, longitudinal data were used to allow for temporal ordering of events and an assessment of causal influences on school completion. The relationship between early drug use and school completion was analyzed separately by ethnicity. The goal was to determine whether substance use in early adolescence produced similar educational outcomes among Latino, African-American, and White males. Data were used from the Rochester Youth Development Study (RYDS), a study of delinquency and drug use among adolescents. The sample consisted of 1,000 students that were 7th and 8th graders during the spring 1988 semester. The results show that, when estimated independently by ethnicity, the relationship between substance use in early adolescence and school completion can be characterized as complex. The impact of early use on the failure to graduate high school was not consistent in separate models of White, African-American, and Latino males. Once a variety of risk factors were held constant, early alcohol and drug use exerted an independent influence on the failure to complete high school for White and African-American males. This finding is consistent with previous research that identifies a causal effect of substance use on school completion. The impact of early substance use appears to be mediated by impregnation for Latinos, suggesting an indirect relationship between substance use and school completion. Teen pregnancy, parenthood, and marriage have been identified as mediators of the relationship between substance use and delinquency in previous studies. 3 tables, 1 appendix, 6 notes, 28 references