To determine how physicians ration limited critical resources, we studied the allocation of intensive-care-unit (ICU) beds during a shortage caused by a lack of nurses. As the bed capacity of the medical ICU decreased from 18 to 8, the percentage of days on which one or more beds were available decreased from 95 to 55 per cent, and monthly admissions decreased from 122 to 95. Physicians responded by restricting ICU admissions to acutely ill patients and reducing the proportion of patients admitted primarily for monitoring. Among patients admitted because of chest pain, the proportion actually sustaining a myocardial infarction increased linearly with the restriction in bed capacity. Although more patients with myocardial infarction were admitted to non-intensive-care areas, there was no increase in mortality. In addition, physicians transferred patients out of the ICU sooner. There was no apparent withdrawal of care from dying patients. Our results suggest that physicians can respond to moderate resource limitations by more efficient use of intensive-care resources.