ABSTRACT. Objective: The aim of this study was to examine the relationship between truancy and escalation of substance use during adolescence and to explore potential mechanisms of this relationship. Method: Using data from the Rochester Youth Development Study, a longitudinal sample of predominantly minority youth, growth models with time-varying covariates were utilized to assess the relationship between truancy and substance use. Mediated growth models were used to examine potential mechanisms of the relationship. The analyses used fi ve waves of panel data collected from 971 youth and their primary caregivers. Data were collected every 6 months from 1988 to 1990, spanning ages 14-16. Twenty-seven percent of the sample was female. Results: Findings indicate that truant youth engaged in more substance use, both when comparing one adolescent with another (i.e., a truant adolescent used more substances than an adolescent who was not truant) and when comparing periods of change within an adolescent (i.e., during periods when an adolescent's truancy escalated, his or her involvement in substance use escalated). Moreover, the effect of escalation of truancy on escalation of substance use was, in part, mediated by escalation of risky, unsupervised time spent with peers. Conclusions: Truancy appears to be a robust predictor of substance use. The effect is likely to be, in part, a result of the deleterious effects of reduced school bonding and, in part, a result of the unsupervised, risky time afforded by truancy. Gaining a better understanding of how truancy may affect substance use is important for the development of prevention and intervention initiatives. (2007a), considered a relatively understudied school-related behavior, truancy, and demonstrated that truancy was a robust predictor of the onset of substance use. Specifi cally, truancy (i.e., skipping school without a valid excuse) predicted the onset of substance use over and above many other salient risk factors, including other school-based risk factors such as school performance and commitment to school. These fi ndings are congruent with other studies that have found a positive relationship between truancy and substance use (Bryant and Zimmerman, 2002;Chou et al., 2006;Vucina and Becirevic, 2007;White et al., 2007). In this study, we extended this work to consider the relationship between truancy and the escalation of substance use. We examine how both level of truancy and change in truancy are related to substance use among urban adolescents. We also explore a probable mediator of the relationship between truancy and escalation of substance use: risky time spent with friends.