Controlled studies provide little empirical evidence to inform clinical recommendations for the optimal duration (i.e., "dosage") of psychosocial treatment for substance use disorders (SUDs). The current study prospectively examined the relationships among treatment dosage, participant adherence to the treatment regimen, and treatment outcomes in a population of adults with stimulant use disorder (cocaine and/or methamphetamine). The study randomly assigned eighty-five participants to receive either 4 weeks or 16 weeks of standardized outpatient treatment. The treatment consisted of cognitive behavioral therapy (CBT) and content covered was identical for each condition; only the planned duration of participation differed. Although both groups reduced stimulant use over time, participants in the 16-week condition were significantly more likely than those in the 4-week condition to provide stimulantnegative urine specimens 26 and 52 weeks following randomization. Participant adherence to treatment correlated significantly with drug-use outcomes: we observed a greater likelihood of stimulant-negative urine tests among those who completed treatment, irrespective of group assignment. Both the number of sessions attended and the percentage of prescribed sessions attended were associated with reductions in stimulant-use frequency 26 and 52 weeks after admission.