This article presents an analysis of logical inconsistencies in adolescents' reporting of recent substance use to assess the potential effect of inaccurate reporting on measures of treatment outcomes and program performance. We used data on 1,463 clients from 10 adolescent treatment programs to assess the relationship between inconsistent reports and various factors that contribute to program assignment and treatment outcomes. Our results suggest that inconsistencies do not arise at random. Instead, inconsistencies are associated with program assignment and factors widely considered to influence treatment outcomes, including age at first use, living situation, race/ethnicity, and mental distress. We also found a positive relationship between level of inconsistent reporting of drug use and self-reports of improvement over time on several well-established treatment outcome measures. Our study highlights the need for greater awareness on the potential impact of inaccuracies in the reporting of substance use on outcome and performance measurements and that for the development of methodologies to improve accuracy.