The purpose of this study was to document the emetogenic potential of intrathecal chemotherapy (IC) in children and to evaluate the efficacy of ondansetron in reducing nausea and vomiting with this chemotherapy treatment. Patients less than 18 years of age with acute lymphoblastic leukemia were eligible to participate in a survey project measuring the emetogenic potential of various chemotherapy treatments. Patients surveyed for 1 or more IC treatments were included in this report. The IC consisted of methotrexate, hydrocortisone and cytarabine, dosed according to patient age. A nausea/vomiting survey instrument was completed by each patient and/or parent following IC treatment. The instrument rated nausea, vomiting and daily activity interference (DAI) on a 4-point scale of 0 = none, 1 = mild, 2 = moderate and 3 = severe, and collected data on the number of vomiting and/or retching episodes in addition to the child's appetite following the chemotherapy treatment. When ondansetron was employed, it was administered in an i.v. infusion at a dose of 0.15 mg/kg before and after chemotherapy or as an oral dose of 4 mg or 8 mg before chemotherapy. Courses of IC without antiemetics were analyzed to determine the emetogenic potential of IC. For patients receiving IC both with and without ondansetron, courses were compared with each patient used as their own control to determine the influence of ondansetron upon survey responses. Statistical analysis consisted of nonparametric Friedman 2-way ANOVA for ordinal variables and a paired t-test for continuous variables. The binomial test was employed to analyze for differences between ondansetron and no antiemetic in the number of patients with complete control of both nausea and vomiting or vomiting alone. A total of 63 children with a mean age of 7.6 +/- 4.2 years were each studied on one or more occasions. Thirty-seven children were surveyed for 87 IC treatments without antiemetics (group I), and 17 children from this group were surveyed for 48 IC courses with i.v. ondansetron (group IA). An additional 18 children were subsequently surveyed for 39 IC courses with i.v. ondansetron (group II). Fifteen patients (7 of whom were members of group I) were surveyed following 33 IC courses with oral ondansetron (group III). The survey scores for group I patients were: nausea severity 1.3 +/- 1.1, vomiting severity 1.2 +/- 1.1, DAI 1.2 +/- 1.0 and mean number of emetic episodes 4.7 +/- 8.4. The mean appetite score was 1.5 +/- 1.1. For patients in group IA, nausea severity (0.8 +/- 0.9), vomiting severity (0.5 +/- 0.8), DAI (0.7 +/- 0.8), and the number of emetic episodes (1.4 +/- 2.8) were all significantly lower than with prior IC treatments without ondansetron. For complete protection, children receiving i.v. ondansetron had greater complete protection rates from both nausea and vomiting or vomiting alone than did patients receiving no antiemetic. Survey responses were also lower for patients receiving oral ondansetron, but insufficient control data did not allow for statistical analysis. IC results in mild to moderate nausea and vomiting in children. The emetogenic potential of IC is significantly reduced by i.v. ondansetron.