Many patients with systemic illness are admitted to intensive care units (ICUs) and placed on life support. This occurs in patient populations even when the projected mortality rate might be as high as 80 to 90%. Patients with systemic illness often are life-support-dependent in ICUs as chronically critically ill patients for many weeks. In order to make decisions concerning withholding or withdrawing life support, more information is needed concerning the survival of ICU patients with systemic illness. Historically, it has been well recognized that critically ill patients who have major organ-system failure have high mortality rates. Studies began to be published in the middle to late 1970s. However, the patient populations studied in different ICUs could not be compared because the evaluation and classification criteria were different. A well-tested and effective classification system that will assist in determining the ICU survival of patients with systemic illness has been developed in the last several years. This system depends upon an acute physiology assessment and chronic health evaluation and is known as the APACHE II system. This generalizable classification system has helped in showing that patients with systemic illness and multisystem organ failure have very high mortality rates. For example, mortality for patients with three or more organ-system failures persisting after 3 days is 98%. Numerous studies have identified that chronologic age is associated with increased mortality rates. However, a better assessment would be biologic age, which would take into account both chronologic age and health status. ICU survival of patients with cancer, hematologic neoplasms, renal failure, liver failure, AIDS, and burns is reviewed.(ABSTRACT TRUNCATED AT 250 WORDS)