The efficacy and safety of ondansetron 8 mg BID compared with 8 mg TID for 3 days in the prevention of nausea and vomiting in 402 patients on cyclophosphamide (≤500 mg/m2)-based chemotherapy we're evaluated in a multicenter, randomized, double-blind, stratified study. The percentage of patients with no emetic episodes over the 3-day study period was 61% in the ondansetron BID group compared with 58% in the ondansetron TID group. Among patients with at least one emetic episode, the mean time to emesis was 14 hr and 17 min in the ondansetron BID group compared with 12 hr and 48 min in the ondansetron TID group. Patients' daily appetite ratings and nausea scores were not significantly different between groups. Clinical laboratory and adverse event profiles were similar between groups. This study is the first large-scale, double-blind trial to demonstrate that ondansetron 8 mg BID for 3 days, a dosing regimen that may enhance patient convenience and compliance, is as effective as ondansetron 8 mg TID for 3 days in the prevention of nausea and vomiting associated with cyclophosphamide-based chemotherapy.