We surveyed 1,353 attending and 689 house staff physicians of the University Hospitals of Cleveland to ascertain the parameters of the CBC, leukocyte differential, and reticulocyte reports perceived as useful in clinical practice. The response rate was 33% for attending and 22% for house staff physicians. Only 4 of 11 parameters routinely reported in the CBC battery were selected as frequently or always useful by more than 90% of physicians: hemoglobin, hematocrit, platelet count, and WBC count. Among primary care physicians, the mean cell volume also attained this level of usefulness for the evaluation of anemia. There were no differences between academic physicians and community physicians in the use of RBC indices; however, physicians who had been in practice for fewer than 10 years indicated higher use of the red cell distribution width than physicians practicing for more than 10 years. Most physicians prefer differentials reported as percentages rather than absolute counts. Among physicians who monitor reticulocyte counts, the immature reticulocyte fraction is not widely used. Our results indicate that many physicians do not use much of the data provided in routine CBC/differential and reticulocyte reports. Some modifications of report formats may facilitate physician perception of hematology laboratory results.