Among severely traumatized patients, infection is second only to head trauma as the leading cause of death. Few studies have defined the infections that occur, the risk factors involved, or the appropriate means of evaluating these patients. In our trauma unit, daily infection surveillance included clinical evaluation of every patient and all microbiologic data. In addition, prophylactic and therapeutic antibiotics were directly under our control. Over a two period 2,368 patients were admitted, most arriving directly from the scene by patients were admitted, most arriving directly from the scene by helicopter. The over-all mortality was 20 percent. In this setting, 639 nosocomial infections occurred in 381 patients of whom 14 percent died of their infection. Sites of infection in percent of total and of bacteremia (given in parentheses) were urinary tract 18 (3), pneumonia 15 (19), empyema 11 (11), phlebitis 12 (17), primary bacteremia 10 (21), surgical wound 19 (8), intraabdominal 8 (11), CNS 7 (5), sinusitis 5 (0), arterial lines 2 (4) and other 3 (1). Over-all 44 percent of infections were bacteremia. Organisms involved in nosocomial infections as percent of total and in bacteremias given in parentheses) were coagulase-positive Staphylococcus 24 (39), other gram-positive cocci 13 (8), Escherichia coli 13 (9), Proteus 4 (5), anaerobes 3 (1) and other organisms 12 (8). Most infections were directly related to an invasive procedure.