Investigations describing the utilization pattern and documenting the value of intensive care are limited by the lack of a reliable and valid classification system. In this paper, the authors describe the development and initial validation of acute physiology and chronic health evaluation (APACHE), a physiologically based classification system for measuring severity of illness in groups of critically ill patients. APACHE uses information available in the medical record. In studies on 582 admissions to a university hospital ICU and 223 admissions to a community hospital ICU, APACHE was reliable in classifying ICU admissions. In validation studies involving these 805 admissions, the acute physiology score of APACHE demonstrated consistent agreement with subsequent therapeutic effort and mortality. This was true for a broad range of patient groups using a variety of sensitivity analyses. After successful completion of multi-institutional validation studies, the APACHE classification system could be used to control for case mix, compare outcomes, evaluate new therapies, and study the utilization of ICUs.