In an open, drug-oriented phase-II/III-study 24 patients were treated with the 5-HT3-antagonist Ondansetron as an antiemetic drug for chemotherapy-induced nausea and emesis. Patients with treatment regimen containing cisplatin were excluded. All patients had suffered from severe nausea and vomiting under conventional antiemetic drugs during a previous identical chemotherapy cycle and were treated with 8 mg Ondansetron t.d. on the day of the chemotherapy and on the four following days. The drug was given with 90 cytostatic cycles ranging from 1 to 14 cycles per patient. Only 2 patients (8%) did not experience an improvement of their symptoms in any of the treatment cycles as measured by a self-conducted grading of nausea and by the frequency of vomiting in comparison to a previous treatment cycle under conventional antiemetic therapy. Eleven out of 18 patients, who were treated with Ondansetron more than once (61%) noted a diminished frequency of vomiting in each treatment cycle with Ondansetron. Sixty of the 90 therapy cycles with Ondansetron resulted in complete (no vomiting) or major (one to two vomits within 24 h following chemotherapy) protection from emesis (37 and 29 per cent, respectively). The most frequent side effect noted was obstipation (7 patients), followed by slight diffuse abdominal pain (4 patients, probably also due to chemotherapy) and slight to severe headache (3 patients, 1 patient was therefore withdrawn from the study). No other side effects were seen. In conclusion, our study indicates that Ondansetron is an effective and safe drug for the treatment of cytostatic drug-induced nausea and vomiting.