Although we commonly assume that because residents spend a given number of months in the emergency department they achieve adequate exposure to all necessary clinical entities, this has never been shown. We suspect, rather, that great variability exists among residents in the number and variety of patients they see; and that with respect to the ED, there are important diagnoses that are rare or absent in the clinical pathology of a training program. To confirm these hypotheses, we implemented a computerized system of recording patients and diagnoses managed in the ED by the 33 residents of the University of Illinois Affilited Hospitals Emergency Medicine Residency. We collected data for nine months and accumulated 2,152 shifts of clinical experience. These data confirm our hypotheses. We found that senior residents managed an average of 11.9 ± 2.3 patients per ten-hour shift, but the quickest resident saw almost twice as many patients as the slowest. Junior residents saw fewer patients, 8.5 ± 1.4 patients per shift, but maintained a twofold difference between the fastest and slowest. Furthermore, there are important diagnoses that present too rarely for each resident to become facile in their management. We found that 22.7% of the 554 diagnoses listed in the Emergency Medicine Core Content never once presented to the ED. An additional 34.7% of these diagnoses did present, but so rarely that each resident could not possibly manage one case during a residency. The Length of Training Report of the American College of Emergency Physicians provides objective guidelines for the number of encounters a resident should have with 283 clinical entities. In this study, residents fell short of these guidelines with 50.5% of diagnoses. While absolute quality of exposure does not assure competence in management, we recommend that each residency monitor the experience of its residents. This allows a residency to change its curriculum to make optimum use of available pathology, as well as to supplement deficiencies in clinical experience with case simulations. © 1990 American College of Emergency Physicians.