APACHE II is a well validated system for rating the severity of medical illness. Its name stands for "Acute Physiology And Chronic Health Evaluation". It is most frequently used to predict in-hospital death for patients in an intensive care unit. Previous scoring systems were based primarily on the patient's current medical state (for instance, rate of breathing or laboratory values). APACHE II incorporates pre-existing medical illnesses into its score (for example, diabetes), which is thought to make it more accurate.

Though well validated, there are published instances where prediction of mortality has not coincided with actual mortality rates, though these might represent special cases. For instance, in a study of gynecological ICU admissions in a Finnish hospital, it was observed that mortality was not predicted by the APACHE II score. In the case of Indian respiratory patients in New Delhi, a study determined that calibration of the APACHE II scoring system to particular populations and conditions might be desirable.^{Need for critical care in gynaecology: a population-based analysis. Critical Care 6, 371-375. PMID 12225615.}

1. ^{⇧ Gupta, Rajnish; and Arora, V.K. (2004). Performance evaluation of APACHE II score for an Indian patient with respiratory problems. Indian Journal of Medical Research 119, 273-282. PMID 15243165.}