PURPOSE: To examine the safety, effectiveness, and patient-perceived benefit of treatment with olanzapine for nausea and vomiting (N/V) in patients with advanced cancer. METHODS: We conducted a multicenter prospective observational study in a tertiary care setting (Trial registration number: UMIN000020493, date of registration: 2016/1/12). We measured the following: average nausea in the last 24 h using a Numeric Rating Scale (NRS: range 0-10) at baseline and day 2, patient-perceived treatment benefit (based on a 5-point verbal scale), and adverse events (AEs; using the Common Terminology Criteria for Adverse Events version 4). RESULTS: The 85 participants (45% men) had a mean age of 58.7±15.8 years. Major causes of N/V were opioids (44%) and chemotherapy (34%). All patients received a daily dose of olanzapine of 5 mg or less as first-line treatment (N=35) or second- or later-line treatment (N=50). Nausea NRS decreased from 6.1±2.2 to 1.8±2.0 (differences: -4.3, 95% CI -3.7 to -4.9, p<0.001). The proportion of patients who did not experience vomiting episodes in the last 24 h increased from 40-89%. Mean decrease in nausea NRS by patient-perceived treatment benefit were as follows: -0.8 for "none" (n=4, 5%); -2.8 for "slight" (n=17, 20%); -3.3 for "moderate" (n=14, 16%); -4.7 for "lots" (n=25, 29%); and -6.1 for "complete" (n=25, 29%; p-for-trend<0.001). The most prevalent AE was somnolence (n=15, 18%). CONCLUSION: Short-term and relatively low-dose olanzapine treatment was effective for multifactorial N/V. Confirmatory studies with longer observation periods are needed to clarify the duration of the effect and adverse events.