The emergency department (ED) provides a substantial amount of critical care. The purpose of this study was to quantitate the critical care provided in an urban ED. The study was performed at a large urban hospital with an annual ED volume of 70,000 patients. All patients requiring critical care in the ED or hospital were prospectively observed between September 1 and November 30, 1993. Patients requiring recovery room care or neonatal intensive care were excluded. Data collected included age, disposition, length of stay, triage acuity assignment in the ED, primary diagnosis, and critical care procedures. During the study, 11,989 patients were examined in the ED with 500 (5%) requiring admission, 340 (3%) requiring a critical care intervention, and 96 (1%) being admitted to an intensive care unit (ICU). Ten percent of patients receiving critical care in the ED were admitted to the floor for lack of bed space. Triage assignment of emergent life-threatening or urgent potentially life-threatening condition predicted critical care need and waiting time to examination. The spectrum of critical care procedures performed in the ED and ICU were similar except for advanced life support intervention, which was more common in the ED (17 versus 3), and arterial line placement, which was more common in the ICU (14 versus 1). The spectrum of diagnoses reflected the age of patients with cardiovascular illnesses (33%), metabolic illnesses (18%), and trauma (16%) common in adults (age > or = 17) and infectious illnesses (46%) or metabolic illnesses (21%) more common in pediatric patients. Overall, 14% of adult critical care, 23% of pediatric critical care, and 15% of all critical care provided in the areas studied occurred in the ED. A significant proportion of critical care is provided in the ED, and triage acuity assignment reflects this need. A significant proportion of critically ill patients was admitted to the floor for lack of bed space, which highlights the financial constraints in urban hospitals such as the one studied.