Smoking, drinking, and illicit drug use are leading causes of morbidity and mortality, both during adolescence as well as later in life. The determination of how well national and local policy and intervention efforts address teen substance use depends largely on the collection of valid and accurate data. Assessments of substance use rely heavily on retrospective self-report measures, but the reliability and validity, however, may be limited by various sources of measurement error. This study utilizes four waves of data from the National Longitudinal Survey of Youth spanning eight years. Results from multiple linear regression analyses showed that the single most consistent variable associated with telescoping was the number of years since the substance was first reported. Time since first report was the single consistent variable and was strongly associated with telescoping in each wave-to-wave comparison for all three substances under study. Implications for policy and research are discussed. (Contains 7 tables.)