This study analyzed six waves of panel data from the National Longitudinal Survey of Youth (NLSY). These analyses were conducted to test the stability of self-reported lifetime use and age of onset. Intraclass correlation coefficients (ICCs) indicated that the stability of age of onset reports decreased with longer time frames between follow-ups. The percentage of youths who had discrepancies in self-reported ever use of alcohol at two-year follow-up ranged from 15% to 35%. Higher discrepancy rates were found for males and younger respondents. Differences in report stability as a function of race/ethnicity were minimal. Questions related to lifetime use and age of onset have implications for the study of lifetime trajectories of use and the timing of prevention programs. (Contains 7 tables.)