Chemotherapy-induced nausea and vomiting (CINV) is a significant toxicity of chemotherapy. Olanzapine is recommended in adult patients for the prevention of CINV but has not been prospectively investigated in children.This investigator-initiated, randomized, open-label trial evaluated olanzapine in children (ages 5-18 years) scheduled to receive the first cycle of highly emetogenic chemotherapy (HEC). All participants received aprepitant, ondansetron, and dexamethasone during and 2 days after chemotherapy. Participants in the study group additionally received oral olanzapine 0.14 mg/kg/day (rounded to the nearest 2.5 mg; maximum, 10 mg) during the chemotherapy block and 3 days postchemotherapy. The primary objective was to compare complete response (CR) rates (no vomiting and no rescue medication) between the groups in the acute, delayed, and overall periods. Nausea comparison and safety evaluation were secondary and additional objectives, respectively. The collection of outcomes and adverse events was performed daily until the completion of the overall period.A total of 240 patients underwent randomization. We performed a modified intention-to-treat analysis on 231 patients (116 in the control group and 115 in the study group). A higher proportion of patients in the olanzapine group achieved CR in the acute period (78% v 59%; P = .001), delayed period (74% v 47%; P < .001) and overall period (64% v 38%; P < .001) than in the control group. The proportion of patients with no nausea was significantly higher in the olanzapine group in the acute period (74% v 52%; P < .001), delayed period (74% v 47%; P < .001), and overall period (64% v 37%; P < .001). Grade 1/2 somnolence was greater in the olanzapine group (35% v 11%; P < .001). There was no grade 3/4 somnolence reported.Olanzapine significantly improved CR rates for vomiting in children receiving the first cycle of HEC.