The authors sought to examine short- and mid-term effectiveness of a group cognitive - behavioral therapy (CBT) in pathological gambling (PG) and to analyze predictors of therapy outcome. Two hundred ninety PG patients participated in the current study, all diagnosed on the basis of Diagnostic and Statistical Manual of Mental Disorders (4th edition) criteria, and were given manualized outpatient group CBT (16 weekly sessions). Specific assessment before and after the therapy and at 1, 3, and 6months follow-up was conducted. Logistic regression and survival analyses were applied. Outpatient group CBT was found to be effective, with abstinence rates of 76.1% by the end of therapy and 81.5% at 6months follow-up. The dropout rate during treatment decreased significantly after the fifth treatment session. Psychopathological distress (p = .040) and obsessive - compulsive symptoms were identified as factors predicting relapse and dropout, respectively. These findings suggest that group CBT is effective for treating pathological gamblers. Several psychopathological and personality traits were identified as outcome predictors.