Purpose: The purpose of this phase III trial was to evaluate the efficacy and safety of regimens containing casopitant, a novel neurokinin-1 receptor antagonist, for the prevention of chemotherapy-induced nausea and vomiting during the first cycle in patients receiving moderately emetogenic chemotherapy (MEC). Patients and Methods: Predominantly female patients (98%) diagnosed with breast cancer (96%) who were chemotherapynaïve and scheduled to receive an anthracycline and cyclophosphamide (AC) -based regimen were enrolled onto this multinational, randomized, double-blind, parallel-group, placebo-controlled clinical trial. All patients received dexamethasone 8 mg intravenously (IV) on day 1 and oral ondansetron 8 mg twice daily on days 1 to 3. Patients were randomly assigned to a control arm (placebo), a single oral dose casopitant arm (150 mg orally [PO] on day 1), a 3-day oral casopitant arm (150 mg PO on day 1 plus 50 mg PO on days 2 to 3), or a 3-day IV/oral casopitant arm (90 mg IV on day 1 plus 50 mg PO on days 2 to 3). The primary end point was the proportion of patients achieving complete response (no vomiting/retching or rescue medications) in the first 120 hours after the initiation of MEC. Results: A significantly greater proportion of patients in the single-dose oral casopitant arm, 3-day oral casopitant arm, and 3-day IV/oral casopitant arm achieved complete response (73%, 73%, and 74%, respectively) versus control (59%; P < .0001). The study did not demonstrate a reduced proportion of patients with nausea or significant nausea in those receiving casopitant. Adverse events were balanced among study arms. Conclusion: All casopitant regimens studied were more effective than the control regimen. Casopitant was generally well tolerated.