In a previous phase I study, olanzapine was demonstrated to be a safe and effective agent for the prevention of delayed emesis in chemotherapy-naïve cancer patients receiving cyclophosphamide, doxorubicin, and/or cisplatin. Using the maximum tolerated dose of olanzapine in the phase I trial, a phase II trial was performed for the prevention of chemotherapy-induced nausea and vomiting in chemotherapy-naïve patients. The regimen was 5 mg/day of oral olanzapine on the 2 days prior to chemotherapy, 10 mg on the day of chemotherapy, day 1, (added to intravenous granisetron, 10 mcg/kg and dexamethasone 20 mg), and 10 mg/day on days 2–4 after chemotherapy (added to dexamethasone, 8 mg p.o. BID days 2 and 3, and 4 mg p.o. BID day 4). Thirty patients (median age 58.5 years, range 25–84; 23 women; ECOG PS 0, 1) consented to the protocol, and all were evaluable. Complete response (CR) (no emesis, no rescue) was 100% for the acute period (24 h postchemotherapy), 80% for the delayed period (days 2–5 postchemotherapy), and 80% for the overall period (0–120 h postchemotherapy) in ten patients receiving highly emetogenic chemotherapy (cisplatin ≥70 mg/m2). CR was also 100% for the acute period, 85% for the delayed period, and 85% for the overall period in 20 patients receiving moderately emetogenic chemotherapy (doxorubicin ≥50 mg/m2). Nausea was very well controlled in the patients receiving highly emetogenic chemotherapy, with no patient having nausea [0 on scale of 0–10, M.D. Anderson Symptom Inventory (MDASI)] in the acute or delayed periods. Nausea was also well controlled in patients receiving moderately emetogenic chemotherapy, with no nausea in 85% of patients in the acute period and 65% in the delayed and overall periods. There were no grade 3 or 4 toxicities and no significant pain, fatigue, disturbed sleep, memory changes, dyspnea, lack of appetite, drowsiness, dry mouth, mood changes, or restlessness experienced by the patients. Complete response and control of nausea in subsequent cycles of chemotherapy (25 patients, cycle 2; 25 patients, cycle 3; 21 patients, cycle 4) were equal to or greater than cycle 1. Olanzapine is safe and highly effective in controlling acute and delayed chemotherapy-induced nausea and vomiting in patients receiving highly and moderately emetogenic chemotherapy.