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An increased volume of patients is associated with improved survival in numerous high-risk medical and surgical conditions. The relationship between the number of patients admitted (hospital volume) and outcome among patients with critical ill-nesses is unknown. Methods We analyzed data from 20,241 nonsurgical patients receiving mechanical ventilation at 37 acute care hospitals in the Acute Physiology and Chronic Health Evaluation clinical information system from 2002 through 2003. Multivariate analyses were performed to adjust for the severity of illness and other differences in the case mix. Results An increase in hospital volume was associated with improved survival among pa-tients receiving mechanical ventilation in the intensive care unit (ICU) and in the hospital. Admission to a hospital in the highest quartile according to volume (i.e.,>400 patients receiving mechanical ventilation per year) was associated with a 37 percent reduction in the adjusted odds of death in the ICU as compared with admis-sion to hospitals in the lowest quartile (≤150 patients receiving mechanical ventila-tion per year, P<0.001). In-hospital mortality was similarly reduced (adjusted odds ratio, 0.66; 95 percent confidence interval, 0.52 to 0.83; P<0.001). A typical patient in a hospital in a low-volume quartile would have an adjusted in-hospital mortality of 34.2 percent as compared with 25.5 percent in a hospital in a high-volume quar-tile. Among survivors, there were no significant trends in the length of stay in the ICU or the hospital