Since the emesis induced by cytotoxic drugs is intractable, and is a possible determinant of a patient's QOL during chemotherapy, the control of this adverse event is essential to complete a course of cancer chemotherapy. The anti-emetic effects of a 5-HT3 antagonist, ondansetron hydrochloride (OND), was evaluated during a course of CDDP-containing chemotherapy. Forty-eight patients with gynecologic carcinoma, respiratory malignancy, or urological cancer were followed throughout their treatment courses. For acute emesis, prophylactic OND was given intravenously before CDDP administration, and OND tablets were used for 4 days from the day following CDDP administration as a measure against delayed emesis. The efficacy of OND gradually decreased for the acute emesis (1st course: 73.8%, 2nd course: 62.8% and 3rd course: 56.7%). The efficacy, however, did not decrease against the delayed emesis. Of patients with a good response, "effective" or "highly effective", in the previous treatment course, over 80% could again obtain a good response in the next treatment course. Adverse events of this anti-emetic treatment were not observed except for mild leukocytosis in one case, and this unexplainable effect abated without any specific treatments. In conclusion, the anti-emetic effects of OND sufficiently prevented the emetogenic action of CDDP throughout the treatment course, with a special importance for successful control in the first treatment course. The additional use of corticosteroid might enhance the effects of OND for female patients.