The P300 component of the ERP (event-related potential) is defined as the most positive peak occurring between 250 and 450 milliseconds after the onset of a stimulus. Previous work has suggested that P300 amplitude may serve as a marker for a dimension of psychiatric disorders associated with disinhibited behavior. Reduced P300 amplitude has been shown in individuals with alcohol and substance use disorders, externalizing childhood behavior problems, Antisocial Personality Disorder, and impulsive aggressive behavior. The present study extended this notion by investigating the usefulness of P300 amplitude as an indicator of treatment compliance in substance dependent individuals. No significant differences were found between treatment program completers and non-completers in psychopathology or impulsive/aggressive behaviors. A significant group difference in P300 amplitude across midline electrode sites (Fz, Cz, and Pz) suggested that treatment program non-completers had an overall lower P300 amplitude than program completers. This study highlights the importance of P300 amplitude as a possible predictor of drug treatment program success.