Despite evidence showing that readiness to change substance use predicts reductions in substance use among treated adolescents, there is little research on changes in thoughts about abstinence and marijuana use during and after treatment. The current study tested whether time-varying changes in adolescents' motivation to abstain and perceived difficulty to abstain from marijuana use hinder the return of regular marijuana use during and after treatment. Monthly data on thoughts about abstinence, marijuana use, and treatment utilization were collected over 6 months from 142 adolescents recruited from intensive outpatient substance use treatment. Results indicated that higher motivation to abstain (but not perceived difficulty) predicted fewer days of marijuana use, over and above the individual's average trajectory of marijuana use, the initial severity of marijuana involvement, and the effects of treatment utilization. Moreover, past-month marijuana use influenced both motivation to abstain and perceived difficulty to abstain. Findings highlight the importance of abstinence-related cognitions as a target of intervention during and after treatment and underscore the importance of considering recovery from substance use disorders as a dynamic process of change over time. (Contains 4 tables, 3 figures, and 6 footnotes.)