The purpose of this research was to examine the effects of cognitive impairment on the efficacy of substance abuse treatment outcome. Alcohol, drug, medical, legal, psychological, employment, and family functioning related treatment outcomes were examined for 26 cognitively impaired and 68 cognitively intact abusing outpatients. Subjects were enrolled in an intensive, 3-week, outpatient program for the treatment of their substance abuse. Subjects were administered a battery of neuropsychological tests prior to treatment onset, and outcome data were obtained at 1, 3, 6, and 12 months posttreatment entry. No significant between-group differences were found on any of the outcome measures, and significant treatment gains were observed across all problem domains in both groups. Subjects' largest improvements were made in the first month of treatment for alcohol, drug, legal, family, and psychological problems. Improvements for employment and medical problems were not observed until 6 months posttreatment. Success across domains was maintained through 12 months follow-up, with the exception of psychological problems; 12-month data indicated a return to thelevel observed at 30 days posttreatment for psychological problems, a level that reflected significant improvement from baseline functioning. A greater proportion of treatment dropouts (i.e., no follow-up data obtained after 30 days) were cognitively impaired as compared to treatment completers. These results suggest that this method of intensive substance abuse outpatient treatment is effective for cognitively impaired patients, an important finding given that research evaluating the efficacy of interventions for such patients is limited. Additionally, neuropsychological evaluation may be important in reducing treatment dropouts, as the present findings indicated that greater cognitive impairment was related to an increased likelihood of treatment dropout.