BACKGROUND: Postdischarge nausea and vomiting after ambulatory surgery is a common problem that is not adequately addressed in current practice. This prospective, randomized, double-blind, parallel-group, placebo-controlled study was designed to test the hypothesis that oral olanzapine is superior to placebo at preventing postdischarge nausea and vomiting. METHODS: In a single-center, double-blind, randomized, placebo-controlled trial, the authors compared a single preoperative dose of olanzapine 10 mg to placebo, in adult female patients 50 years old or less, undergoing ambulatory gynecologic or plastic surgery with general anesthesia. All patients received standard antiemetic prophylaxis with dexamethasone and ondansetron. The primary composite outcome was nausea and/or vomiting in the 24 h after discharge. Secondary outcomes included severe nausea, vomiting, and side effects. RESULTS: A total of 140 patients were randomized and evaluable. The primary outcome occurred in 26 of 69 patients (38%) in the placebo group and in 10 of 71 patients (14%) in the olanzapine group (relative risk, 0.37; 95% CI, 0.20 to 0.72; P = 0.003). Severe nausea occurred in 14 patients (20%) in the placebo group and 4 patients (6%) in the olanzapine group (relative risk, 0.28; 95% CI, 0.10 to 0.80). Vomiting occurred in eight patients (12%) in the placebo group and two patients (3%) in the olanzapine group (relative risk, 0.24; 95% CI, 0.05 to 1.10). The median score for sedation (scale 0 to 10, with 10 being highest) in the 24 h after discharge was 4 (interquartile range, 2 to 7) in the placebo group and 6 (interquartile range, 3 to 8) in the olanzapine group (P = 0.023). CONCLUSIONS: When combined with ondansetron and dexamethasone, the addition of olanzapine relative to placebo decreased the risk of nausea and/or vomiting in the 24 hafter discharge from ambulatory surgery by about 60% with a slight increase in reported sedation.