From a convenience sample of 500 consecutive patients seen in the emergency department (ED), occasions were recorded when data from the Veterans Affairs Decentralized Hospital Computer Program provided immediate clinical decision support and obviated redundant laboratory tests. Patient care was improved by access to inpatient discharge summaries in 85 cases (19%), laboratory results in 34 (7%), pharmacy records of allergies and prescriptions in 30 (6%), radiologic reports in 19 (4%), and electrocardiograms in 11 (2%). Overall savings in tests, prescriptions, admissions, and errors were estimated at about $5 per visit. Availability of previous laboratory results clearly decreased ordering of redundant studies. Computer-based medical records also provided details of previous diagnoses, treatments, allergies, and current medications. On many occasions, the presumptive diagnosis had already been worked up and proven or disproven, thus simplifying the entire encounter.