The APACHE (Acute Physiology and Chronic Health Evaluation) prognostic scoring system was developed in 1981 at the George Washington University Medical Center as a way to measure disease severity. APACHE II, introduced in 1985, was a simplified modification of the original APACHE. The APACHE II score consisted of three parts: 12 acute physiological variables, age and chronic health status. Probability of death can be derived by using the disease category and the APACHE II score. The uses of APACHE II include risk stratification to account for case mix in clinical studies, comparison of the quality of care among ICUs, and assessment of group and individual prognoses. APACHE III, a refinement of APACHE II, will be introduced in late 1990. The APACHE III data base includes 17,457 patients from a representative sample of 40 American hospitals. Additional potential uses of APACHE III include the identification of factors in the ICU which contribute to outcome and assistance in individual patient decision-making. This article reviews the development, current uses and potential applications of the APACHE system.