Purpose: Aprepitant, a 5-HT3 receptor antagonist (5HT3-RA), and dexamethasone are standard antiemetic therapy for prevention of single-day, cisplatin-induced nausea and vomiting. We conducted a double-blind, placebo-controlled phase III cross-over study that compared aprepitant to placebo combined with standard antiemetic prophylaxis (a 5HT3-RA and dexamethasone) in patients receiving 5 days of cisplatin combination chemotherapy for testicular cancer. Patients and Methods: Patients receiving two consecutive identical courses of a 5-day cisplatin-based chemotherapy were randomly assigned to aprepitant 125 mg on day 3 and 80 mg per day on days 4 through 7 or to placebo with the initial course and crossover to the opposite treatment with the second course. The primary objective was complete response (CR). Secondary end points were emetic episodes (acute and delayed), nausea measurement based on a visual analog scale (VAS), and patient-stated preference after the second study cycle. Results: In all, 71 patients were screened for the study and 69 were evaluable. Thirty-five patients were randomly assigned to receive aprepitant and 34 to receive placebo for the first course. Forty-two percent achieved CR with aprepitant compared with 13% with placebo (P < .001). Eleven patients (16.2%) had at least one emetic episode during the aprepitant cycle versus 32 patients (47.1%) with placebo. Thirty-eight patients preferred the aprepitant cycle whereas 11 preferred placebo (P < .001). There was no statistical difference in VAS for nausea, but it was numerically superior with aprepitant. There was no toxicity with aprepitant compared with placebo. Conclusion: There was a significant improvement in CR rate with aprepitant combined with a 5HT3-RA and dexamethasone. Patient preference strongly favored the aprepitant cycle.