OBJECTIVE: To evaluate the impact of computerized ordering guidelines on clinician ordering patterns for neuroradiology imaging studies of the head. DESIGN: A retrospective analysis was performed using a pre-post design. A 9-week control period was followed by an 8-week intervention period. SUBJECTS: All clinicians who placed an order for either an MRI of the brain or a CT of the head on inpatients using a computerized order entry system. METHODS: We designed, implemented, and evaluated a decision support system for the implementation of test ordering guidelines. Changes in ordering patterns were evaluated with a Chi-square analysis. RESULTS: 742 tests were ordered in the pre-intervention period, while 704 studies were ordered after the intervention. A significant change in the distribution of tests ordered resulted from the intervention (p=0.048). Changes trended toward the guideline recommendations for all tests considered. 60% of users receiving a recommendation ordered the suggested study. DISCUSSION: Our intervention successfully influenced clinician ordering patterns. Examination of detailed usage patterns may aid in further quality improvement of both the guidelines and the decision support tool used to implement them.