Abstract This study attempted to evaluate the ability of an outpatient drug rehabilitation program to effect significant shifts on the five major dimensions of personality. A mostly African American sample of 82 men and 50 women entering a 6-week program were assessed at admission, and the 99 who completed were again measured at termination. Follow-up assessments were completed on 30 clients an average of 15 months later. Results indicated significant shifts on all five personality domains from pre- to posttreatment (mean Cohen's d = .38). Significant shifts on Neuroticism, Agreeableness, and Conscientiousness were maintained over follow-up (mean Cohen's d = .28). These results suggest that personality change may be possible in the context of treatment.