oBjective: To determine if increasing use of ondansetron is associated with improved clinical outcomes. design/Methods: A retrospective cohort study was conducted at The Hospital for Sick Children (Toronto, ON). Eligible children included those <18 years of age who presented to the emergency department (ED) between July 2003 and June 2008 and were diagnosed with gastroenteritis. 20% of all eligible patients were selected at random for chart review. The primary outcome was the correlation over

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... me between the proportions of children administered ondansetron and those that received intravenous rehydration. A time trend analysis comparing 2003-05 vs. 2006-08 was conducted using intravenous rehydration as the outcome. A structural model was built using unobserved components and the explanatory variable -ondansetron. Secondary outcomes included length of stay, ED revisits and need for intravenous rehydration upon revisit. results: 22,125 potentially eligible visits were identified and 4,425 charts were reviewed. There was a trend towards increasing acuity during the 5 year period as reflected by lower CTAS scores (P<0.001). Over the course of the 5-year study period, there was a reduction in the use of intravenous rehydration. from 27% to 13% (P<0.001) and an increase in ondansetron utilization from 1% to 18% (P<0.001). The time series analysis revealed that seasonality was a major determinant in intravenous rehydration rates, and that there were statistically significant downward trends and downward level breaks (all P<0.001). The porportion of children administered ondansetron was a negative predictor of the need for intravenous rehydration (P<0.001). A reduction in the mean length of stay occurred over this time period (8.6 hours to 5.9 hours; P=0.03). During the week following the index visit there was a reduction in return visits to our ED (18% to 13%; P=0.008), and need for intravenous rehydration (7% to 4%; P=0.02). conclusions: Ondansetron use has increased significantly. This is associated with reductions in intravenous rehydration and ED revisits and a shorter length of stay for children with gastroenteritis. The selective use of ondansetron in the treatment of children with gastroenteritis results in improved clinical outcomes.