BACKGROUND AND OBJECTIVE: Computed tomography (CT) and ultrasound (US) are commonly used in patients with acute abdominal pain. We sought to standardize care and reduce CT use while maintaining patient safety through implementation of a multicomponent electronic clinical decision support tool for pediatric patients with possible appendicitis. METHODS: We conducted a quasi-experimental study of children 3 to 18 years old who presented with possible appendicitis to the pediatric emergency department (ED) between January 2011 and December 2013. Outcomes were use of CT and US. Balancing measures included missed appendicitis, ED revisits within 30 days, appendiceal perforation, and ED length of stay. RESULTS: Of 2803 patients with acute abdominal pain over the 3-year study period, 794 (28%) had appendicitis and 207 (26.1% of those with appendicitis) had a perforation. CT use during the 10-month preimplementation period was 38.8% and declined to 17.7% by the end of the study (54% relative decrease). For CT, segmented regression analysis revealed that there was a significant change in trend from the preimplementation period to implementation (monthly decrease -3.5%; 95% confidence interval: -5.9% to -0.9%; P = .007). US use was 45.7% preimplementation and 59.7% during implementation. However, there was no significant change in US or total imaging trends. There were also no statistically significant differences in rates of missed appendicitis, ED revisits within 30 days, appendiceal perforation, or ED length of stay between time periods. CONCLUSIONS: Our electronic clinical decision support tool was associated with a decrease in CT use while maintaining safety and high quality care for patients with possible appendicitis.