Since sepsis is the most frequent single cause of death after surgery and trauma, its development should be anticipated in elderly patients or those with disease or trauma causing intestinal leaks, particularly if the patient had massive transfusions or was in shock. Diagnosis may be extremely difficult, particularly if the infection is intraperitoneal. Furthermore, patients with impaired host defenses may show only a failure to thrive and then a progressive MOF. Physical examination is usually not very helpful. Gallium and indium scans and ultrasonography are only about 50 to 60 per cent accurate. Ultrasonography followed by HIDA and PIPIDA scans may be very useful in diagnosing acute acalculous cholecystitis, which appears to be an increasingly frequent problem in these patients. Computerized tomographic scans are at least 80 to 90 per cent accurate in diagnosing intra-abdominal abscesses, but the diagnosis of peritonitis is still largely clinically based. Even without clear evidence of infection, the critically ill patient with MOF and previous abdominal trauma, surgery, or disease should probably have the abdomen explored (that is, a blind laparotomy). If generalized peritonitis is found, it may be wise to leave the abdomen open and re-explore and débride it daily until it is clean. Percutaneous drainage of abdominal abscesses is being performed increasingly and is of special value in the 30 to 50 per cent of patients with single bacterial abscesses in which the drainage tract does not cross bowel or peritoneum and there is no underlying intestinal leak. Antibiotics are only a second line of defense, and their use should be directed by smear and culture results when possible. For abdominal infections, coverage for gram-negative anaerobes and Bacteroides fragilis is essential. If the infection persists for more than 2 to 3 weeks, infection by enterococci and fungi must be considered. If shock develops, maintaining an O2 consumption of at least 130 to 160 ml per minute per m2 is a particularly important part of the resuscitation. Although controversial, raising the hematocrit to 40 to 45 per cent or higher is often of value.