The antiemetic response and side effects resulting from treatment with methylprednisolone (MPA) given on two different dose schedules were evaluated in 20 women with breast cancer who were undergoing chemotherapy consisting of cyclophosphamide, methotrexate and 5-fluorouracil (CMF). This randomized, crossover, double-blind study compared the antiemetic efficacy of a single dose of 125 mg MPN with that of two such doses. The study demonstrated the superiority of the latter protocol in preventing CMF-induced nausea and vomiting. The rate of antiemetic response to single vs double doses was as to follows: complete protection, 17% vs 30%; partial and minimal protection, 39% vs 55%; and no protection, 44% vs 15% of the courses, respectively (P = 0.0087). No difference in the antiemetic response rate was found between the first and the second course. Treatment with MPN was well tolerated, and no difference in the incidence of side effects was found between the single-dose and the double-dose schedule. We recommend the use of two doses of 125 mg MPN as prophylactic antiemetic treatment in breast-cancer patients receiving CMF chemotherapy.