The field of high-dose chemotherapy with stem cell transplantation has been expanded recently as a treatment for solid tumors and hematological malignancies. Severe emesis remains one of the main extramedullary side-effects of high-dose regimens during the first week of treatment. Traditional antiemetics such as chlorpromazine, diazepam, and phenothiazines are extensively used but are unable to control emesis. The new antiemetic ondansetron, a serotonin receptor (5HT3) antagonist appears to be superior to these drugs for cisplatin-induced emesis. The study we present here is an attempt to control emesis following high-dose regimens, during bone marrow or peripheral stem cell transplantation, with ondansetron. To our knowledge no other paper has reported the efficacy of this antiemetic in such group of patients. A total of 29 patients who received highly emetogenic polychemotherapy as conditioning regimens for bone marrow transplantation were treated with ondansetron, which was given as an 8-mg i.v. short infusion prior the initiation of treatment and every 6 h thereafter for 3 days, and an 8-mg dose every 8 h for 5 additional days. All the patients had previously been treated with chemotherapy and were evaluable for response and toxicity. Complete and major protection of vomiting on day 1 was achieved by 76% of the patients, 58% on day 2 and 52% on day 3. Nausea was absent or mild in 79% of patients on day 1, 45% on day 2 and 41% on day 3.(ABSTRACT TRUNCATED AT 250 WORDS)