A multidisciplinary concurrent audit of the quality of medical care within a trauma system was carried out by a committee of physicians, nurses, and health officials representing trauma centers, nontrauma hospitals, and the public agency administrating the trauma system. Care was audited with regard to timeliness and appropriateness of diagnosis and therapy. Complications were classified as being due to an error in diagnosis, judgment, or technique. Deaths were classified as nonpreventable, potentially salvageable, or frankly preventable. During the first 22 months of operation (1 August 1984-30 May 1986), 7,936 cases were audited. Of the 224 deaths occurring at nontrauma hospitals, 17 (7.6%) were felt to be frankly preventable. Of 541 deaths occurring at trauma centers, 11 (2.0%) were felt to be frankly preventable (p less than 0.001). The most common problem implicated in preventable deaths at nontrauma center hospitals was an error in diagnosis. Preventable deaths at trauma centers were most commonly due to an error in technique. Complications or protocol violations occurred in 595 of 6,564 surviving trauma patients (9.1%). During the first 12 months of system operation, 7,200 person-hours were required to perform the audit. Personnel costs alone for audit in the first year were $300,420.