Stress ulcers are a frequently encountered problem in critically ill medical patients. Gastric acid and decreased gastric mucosal blood flow appear to be important in the pathogenesis of these lesions. Occult bleeding from stress ulcers is common, although significant bleeding occurs in less than 20% of patients. The mortality of bleeding is dependent on the severity of the underlying diseases. A number of processes have been suggested as risk factors; however, prolonged mechanical ventilation, a coagulopathy, and the presence of more than one risk factor place the patient at greatest risk. Titration of the gastric pH to greater than four with either antacids or H2-receptor antagonists provides effective prophylaxis. The continuous infusion of the H2-receptor antagonists is also efficacious. Sucralfate appears to be another useful alternative with several potential advantages. Prophylactic therapy decreases the incidence of stress ulcer-related bleeding, although it does little to improve the survival of the critically ill patient.