BACKGROUND: Olanzapine is an inexpensive and durable agent for the treatment of chemotherapy-induced nausea and vomiting and is also superior to neurokinin-1 receptor antagonists in the control of nausea. This study aimed to investigate the efficacy and safety of a low dose of 5 mg olanzapine plus granisetron and dexamethasone for treatment of carboplatin (CBDCA)-induced nausea and vomiting in patients with thoracic malignancies. MATERIALS AND METHODS: We conducted a prospective, open-label, single-arm, multicenter, phase II trial in four centers in Japan. Registered patients were scheduled to receive area under the curve (AUC) ≥5 mg/mL per minute of CBDCA and had never received moderately to highly emetogenic chemotherapy. Patients received olanzapine 5 mg/day orally after supper for 4 days, in combination with granisetron and dexamethasone. Primary endpoint was complete response (CR; no emesis and no use of rescue medication) rate during the overall phase (0-120 hours). RESULTS: Between February 2018 and June 2020, 51 patients were enrolled, and 50 patients were evaluated. The CR rates in the overall (0-120 hours), acute (0-24 hours), and delayed phases (24-120 hours) were 94.0%, 100%, and 94.0%, respectively. No grade 3 or higher adverse effects of olanzapine were observed. CONCLUSIONS: Prophylactic antiemetic therapy with a low dose of 5 mg olanzapine plus granisetron and dexamethasone showed durable efficacy with an acceptable safety profile. This three-drug combination appears to be a reasonable treatment approach in patients with thoracic malignancies receiving an AUC ≥5 mg/mL per minute of CBDCA-based regimen. Clinical trial identification number. UMIN000031267. IMPLICATIONS FOR PRACTICE: The results of this phase II trial indicated that the prophylactic administration of low-dose olanzapine (5 mg) combined with granisetron and dexamethasone has promising activity with acceptable safety profile in patients with thoracic malignancy receiving high-dose carboplatin chemotherapy.