It is of the utmost importance that those developing prevention and intervention programs in reducing juvenile offending and youth violence be informed by empirical research studies examining longitudinal relations between substance use and delinquency. This study documents the extent to which problem behaviors co-occur in early adolescence, how relations between them change over time, and the degree to which these behaviors serve to maintain or accelerate each other. Participants in the study were members of a longitudinal study of middle school students and their families conducted in South Florida. Data were collected beginning when students were in the sixth and seventh grades and annually for 3 years. Students completed questionnaires at school yearly. The final sample for the study consisted of all students with complete questionnaire data for all 3 interviews (n=5,045). Results document significant changes in the co-patterning of substance use and delinquent behavior in a multiethnic community sample of middle school students. As participants aged, alcohol use and delinquent behavior became increasingly normative, and illicit substance use also increased significantly in prevalence. The findings suggest an acceleration effect for indices of each problem behavior when they co-occur. Co-occurring problem behaviors were associated with significantly higher mean levels of each behavior, with increasing group differences over time. Tables, references