A combination of aprepitant, a 5-HT3 receptor antagonist, and dexamethasone is recommended for the prophylaxis of acute or delayed emesis induced by chemotherapy containing anthracyclines plus cyclophosphamide in patients with breast cancer. The aim of this study was to verify whether dexamethasone is superior to aprepitant in preventing delayed emesis in patients receiving the same prophylaxis for acute emesis. A randomized double-blind study comparing aprepitant versus dexamethasone was completed in chemotherapy-naive patients with breast cancer treated with anthracyclines plus cyclophosphamide. Before chemotherapy, all patients were treated with intravenous palonosetron 0.25 mg, dexamethasone 8 mg, and oral aprepitant 125 mg. On days 2 and 3, patients randomly received oral dexamethasone 4 mg twice per day or aprepitant 80 mg once per day. Primary end point was rate of complete response (ie, no vomiting or rescue treatment) from days 2 to 5 after chemotherapy. Of 580 enrolled patients, 551 were evaluable: 273 received dexamethasone, and 278 received aprepitant. Day 1 complete response rates were similar: 87.6% for dexamethasone and 84.9% for aprepitant (P < .39). From days 2 to 5, complete response rates were the same with both antiemetic prophylaxes (79.5%; P < 1.00), as were results of secondary end points (ie, complete protection, total control, no vomiting, no nausea, score of Functional Living Index-Emesis; P < .24). Incidences of insomnia (2.9% v 0.4%; P < .02) and heartburn (8.1% v 3.6%; P < .03) were significantly greater with dexamethasone on days 2 to 5. In patients with breast cancer treated with anthracycline plus cyclophosphamide chemotherapy and receiving the same antiemetic prophylaxis for acute emesis, dexamethasone was not superior to aprepitant but instead had similar efficacy and toxicity in preventing delayed emesis.