Evaluated the long-term stability of the Child Behavior Checklist (CBCL) in a longitudinal clinical sample of youth with attention deficit hyperactivity disorder (ADHD), testing the hypothesis that the CBCL scales will show stability over time. Participants were 105 Caucasian, non-Hispanic boys with ADHD between the ages of 6 and 17 assessed at baseline and at a 4-year follow-up. Stability of CBCL scales were computed for dimensional (intraclass correlation coefficients [ICCs], Pearson correlations) and dichotomized scale scores (kappa coefficients and odds ratios [ORs]). Evidence was found for stability of the categorical and dimensional types of scores, as demonstrated by statistically significant stability of the Pearson correlation coefficients, kappas, and ORs. The robust findings obtained from ICCs and kappa coefficients document substantial stability for CBCL scales over time within individuals with ADHD. These results support the informativeness of the CBCL as a useful measure of longitudinal course in clinical samples of youth with ADHD.