The antiemetic effectiveness of haloperidol plus dexamethasone was compared with that of prochlorperazine plus dexamethasone in a prospective study of patients receiving chemotherapy for breast cancer. Chemotherapy consisted of cyclophosphamide, doxorubicin or methotrexate, and fluorouracil in all patients. Patients who received the doxorubicin-containing combination experienced significantly more nausea and vomiting than those who received the chemotherapy combination containing methotrexate. There was no significant difference between the two antiemetic regimens in the overall incidence of postchemotherapy nausea and vomiting, but patients who received haloperidol and dexamethasone did have a lower incidence of severe vomiting. Neither regimen is highly effective, especially for the combination containing doxorubicin, and cannot be recommended as standard antiemetic therapy for this population of patients.