Overutilization of laboratory procedures is a significant problem, particularly in teaching institutions. The purpose of this study was to assess laboratory utilization patterns on the medical service of a university hospital, comparing data from implicit peer review by a pathologist with those from a group of practicing primary care physicians. Also, utilization of the most frequently ordered laboratory tests was compared with that of less common procedures. For charts reviewed both by the pathologist and by a panel of primary physicians, the pathologist found 26.5 per cent of tests unnecessary, while the primary physicians found 42.8 per cent unnecessary. The ten most frequently ordered tests had a significantly higher rate of overutilization than all other tests. Attempts to modify physicians' laboratory utilization should include concomitant peer review of the most commonly ordered procedures.