Patients presenting to a community hospital emergency department were studied retrospectively to assess the determinants of diagnostic test ordering, and prospectively following an intervention to reduce test utilization. Records of thirty patients were selected for detailed physician review. Consensus criteria for excessive testing were defined by four physician reviewers and applied to each record. One third of all tests were judged excessive. Of multiple demographic and clinical variables studied, only patient age and severity were related to test ordering behavior. Six months after an administrative change caused by review of these findings, a repeat audit of sixty patient records revealed significantly fewer tests ordered (p less than .05) and significantly fewer excessive tests (p less than .05). The projected annual savings in laboratory charges for these patients were $14,838. Simple distribution of the base-year findings, by assisting a change in policy, have effected substantial reductions in excessive laboratory testing.