Using a quasi-experimental design, this study examined longitudinal outcomes for families previously enrolled in a study of Parent-Child Interaction Therapy (PCIT), a treatment program for young children with disruptive behavior disorders. Comparisons were made between 23 families who completed treatment and 23 families who dropped out of the study before completing treatment, using a structured diagnostic interview, and several parent-report measures. Length of follow-up for both groups ranged from 10 to 30 months after the initial assessment, with the average length of follow-up just under 20 months. Results indicated consistently better long-term outcomes for those who completed treatment than for study dropouts. These results highlight the need to identify salient predictors of treatment engagement and retention to maximize outcomes for young children.