Study Objective: Vomiting in children suffering from acute gastroenteritis interferes with the oral rehydration process and equally frustrates parents and health care providers. Adjuncts such as promethazine and metoclopramide are less than optimally effective and are associated with side effects. Ondansetron, a 5-HT₃ receptor antagonist marketed as Zofran, is a safe and effective antiemetic used extensively in oncology and postoperative patients. We evaluate the effect of the antiemetic ondansetron versus placebo on the clinical outcome of patients with vomiting from gastroenteritis in a pediatric emergency department. Methods: This was a randomized, prospective, double-blind clinical trial in a university-affiliated children's hospital ED. Children between the ages of 6 months and 12 years who had vomited at least 5 times during the preceding 24 hours were randomized to receive either oral ondansetron or a taste- and color-matched placebo. Oral rehydration was commenced 15 minutes later at 5 mL/min per standard oral rehydration protocols. Patients were discharged after they voided and continued standard oral rehydration at home with the introduction of a bananas, rice, applesauce, and toast (BRAT) diet after the first 24 hours. Any patient requiring admission was considered a treatment failure, and no further doses were given. Discharged patients were given 5 additional doses to be used every 8 hours, and they were contacted by telephone 24 and 48 hours after discharge to record the number of episodes of vomiting and diarrhea. The parents were also required to complete a diary of the same information, which was mailed to the investigators for confirmation of the telephone data. Results: One hundred forty-five patients were enrolled, of whom 51% (n=74) were randomized to ondansetron. At baseline, age distribution, sex, and severity of illness did not differ between the ondansetron and placebo groups. During the observation period in the ED, the median number of episodes of vomiting was 0 in both groups, but the rank sum of vomiting episodes was significantly lower in the ondansetron group (P =.001). The number of episodes of emesis in the ED after enrollment ranged from 0 to 7 in the placebo group and 0 to 2 in the ondansetron group. During the 48 hours of follow-up, the median number of episodes of vomiting remained 0, with no statistically significant difference between the groups. There was no statistically significant difference in the rank sum of episodes of diarrhea in the ED between the groups (P =.622); however, during the next 48 hours, the patients in the ondansetron group had significantly more diarrhea than the placebo group. A lower proportion of patients receiving ondansetron compared with placebo required intravenous fluid therapy (P =.015). The admission rate was also lower in patients receiving ondansetron (P =.007). The revisit rate was higher in the ondansetron group compared with the placebo group (P =.047). Conclusion: Ondansetron was effective in reducing the emesis from gastroenteritis during the ED phase of oral rehydration and in lowering the rates of intravenous fluid administration and hospital admission. [Ramsook C, Sahagun-Carreon I, Kozinetz CA, Moro-Sutherland D. A randomized clinical trial comparing oral ondansetron with placebo in children with vomiting from acute gastroenteritis. Ann Emerg Med. April 2002;39:397-403.]