OBJECTIVE: Pathological gambling is associated with bipolar disorder and dopamine dysfunction. Olanzapine is a second-generation antipsychotic with mood-stabilizing properties and antagonistic activity at several dopamine receptors. The purpose of this study was to evaluate olanzapine in the treatment of pathological gambling. METHOD: In this 12-week, single-center, randomized, double-blind, placebo-controlled, flexible-dose (2.5-15 mg/day) trial, 42 outpatients with pathological gambling by DSM-IV-TR criteria received olanzapine (N = 21) or placebo (N = 21). The primary outcome measure was the Pathological Gambling Adaptation of the Yale-Brown Obsessive Compulsive Scale (PG-YBOCS). The primary analysis of efficacy was a longitudinal analysis of the intent-to-treat sample, with treatment-by-time interaction as the effect measure. Subjects were enrolled from June 2, 2000, through November 28, 2005. RESULTS: Compared with placebo, olanzapine was associated with a similar rate of reduction in total scores on the PG-YBOCS scale, as well as in gambling episodes/week, hours gambled/week, and Clinical Global Impressions-Severity of Illness scale scores. The mean (SD) olanzapine daily dose at endpoint evaluation was 8.9 (5.2) mg/day. Eleven subjects (52%) receiving olanzapine and 6 (29%) receiving placebo discontinued prematurely; 3 subjects receiving olanzapine and 2 receiving placebo discontinued because of adverse events. Events causing olanzapine discontinuation were pneumonia, sedation, and hypomania. CONCLUSION: Olanzapine was not superior to placebo in the short-term treatment of pathological gambling. It was also associated with a high discontinuation rate. TRIAL REGISTRATION: ClinicalTrials.gov identifier NCT00438776 (http://www.clinicaltrials.gov).