Purpose: This is the first study in which the NK1-receptor antagonist, aprepitant (APR), was evaluated for the prevention of chemotherapy-induced nausea and vomiting (CINV) with moderately emetogenic chemotherapy. Patients and Methods: Eligible breast cancer patients were naive to emetogenic chemotherapy and treated with cyclophosphamide ± doxorubicin or epirubicin. Patients were randomly assigned to either an aprepitant regimen (day 1, APR 125 mg, ondansetron (OND) 8 mg, and dexamethasone 12 mg before chemotherapy and OND 8 mg 8 hours later; days 2 through 3, APR 80 qd) or a control regimen (day 1, OND 8 mg and dexamethasone 20 mg before chemotherapy and OND 8 mg 8 hours later; days 2 through 3, OND 8 mg bid). Data on nausea, vomiting, and use of rescue medication were collected with a self-report diary. The primary efficacy end point was the proportion of patients with complete response, defined as no vomiting and no use of rescue therapy, during 120 hours after initiation of chemotherapy in cycle 1. The secondary end point was the proportion of patients with an average item score higher than 6 of 7 on the Functional Living Index-Emesis questionnaire. Results: Of 866 patients randomized, 857 patients (99%) were assessable. Overall complete response was greater with the aprepitant regimen than with the control regimen (50.8% v 42.5%; P = .015). More patients in the aprepitant group reported minimal or no impact of CINV on daily life (63.5% v 55.6%; P = .019). Both treatments were generally well tolerated. Conclusion: The aprepitant regimen was more effective than the control regimen for prevention of CINV in patients receiving both an anthracycline and cyclophosphamide.