We evaluated the influence of psychotherapy attendance on treatment outcome in 90 dually (cocaine and heroin) dependent outpatients who completed 70 days of a controlled clinical trial of sublingual buprenorphine (16 mg, 8 mg, or 2 mg daily, or 16 mg every other day) plus weekly individual standardized interpersonal cognitive psychotherapy. Treatment outcome was evaluated by quantitative urine benzoylecgonine (BZE) and morphine levels (log-transformed), performed three times per week. Repeated-measures linear regression was used to assess the effects of psychotherapy attendance (percent of visits kept), medication group, and study week on urine drug metabolite levels. Mean psychotherapy attendance was 71% of scheduled visits. Higher psychotherapy attendance was associated with lower urine BZE levels, and this association grew more pronounced as the study progressed (p=0.04). The inverse relationship between psychotherapy attendance and urine morphine levels varied by medication group, being most pronounced for subjects receiving 16 mg every other day (p=0.02). These results suggest that psychotherapy can improve the outcome of buprenorphine maintenance treatment for patients with dual (cocaine and opioid) dependence.