The objective of this study was to develop criteria to demarcate patients at risk for catastrophic deterioration (arrest or major decompensation) and those likely to require intensive care. From an inception cohort of patients admitted to the medical service, 544 patients were evaluated prospectively for severity of illness and stability by the admitting residents; the course of patients was reviewed blindly by observers. Patients admitted with acute dyspnea, particularly those with chronic pulmonary disease, were at a significantly greater (p less than .01) risk of arrest. All but one of the other arrests occurred in patients who were rated unstable on admission and who had further deterioration of pre-existing problems in the hospital (p less than .0001). The deterioration rates were highest among patients rated as unstable, particularly in patients with comorbid disease. Patients who are unstable on admission or who begin to deteriorate due to comorbid disease or the condition leading to admission, should be considered at extremely high risk for subsequent arrest and should be admitted to critical care units for early observation.