To evaluate the causes, risk factors, and mortality rates associated with unexpected readmission to medical and surgical ICUs. MEDLINE citation review of primary articles focusing on ICU readmission or ICU outcomes from January 1966 to June 1999, and contact with authors of primary studies. Eight primary studies of ICU readmission and eight multi-institutional ICU outcome studies that reported ICU readmission rates were included. We abstracted data on the methodology and design of the primary studies, overall rates, causes, predictors, outcomes, and measures of quality of care associated with ICU readmission. The average ICU readmission rate of 7% (range, 4 to 14%) has remained relatively unchanged in both North America and Europe. Respiratory and cardiac conditions were the most common (30 to 70%) precipitating cause of ICU readmission. Patients readmitted to ICUs had average hospital stays at least twice as long as nonreadmitted patients. Hospital death rates were 2- to 10-times higher for readmitted patients than for those who survived an ICU admission and were never readmitted. Predictors of ICU readmission have been neither well studied nor reproducible. Unstable vital signs, especially respiratory and heart rate abnormalities, and the presence of poor pulmonary function at time of ICU discharge appear to be the most consistent predictors of ICU readmission. There were no consistent data supporting the use of readmission rates as a measure of quality of care. ICU readmission is associated with dramatically higher hospital mortality. Unstable vital signs at the time of ICU discharge are the most consistent predictor of ICU readmission. Further studies focusing on processes of ICU and hospital care are needed to determine if ICU readmission rates are a measure of quality of care.