The stability of self-reports refers to the consistency of self-reported use across several years. This study used fives waves of data across 8 years from the National Longitudinal Survey of Youth. Analyses were conducted to examine the internal or within-wave consistency as well as external or across-waves consistency for self-reported marijuana use. Further tests were conducted to identify if there were any differences for age, ethnicity, and sex for report consistency. Report stability was higher for lifetime use reports than the age of onset reports. Wave-by-wave differences revealed stability remained at acceptable levels in nearly all comparisons at agreement being about 75 percent. Overall, report agreement was higher for females, older adolescents, and non-Hispanic/non-black youths in bivariate analyses. However, only older chronological age remained consistently significantly associated with better report stability in multiple logistic regression models. Implications regarding misclassification of users for prevention programs and measurement issues are discussed. (Published Abstract)