Physicians and surgeons have long recognized that septic illness may be accompanied by abnormal brain functions; however, no systematic, comprehensive study has been done to define the clinical and laboratory features of the syndrome of sepsis-associated encephalopathy. We undertook such a prospective study in a tertiary care hospital and found that of 69 patients with fever and microbial cultures, 32 had marked brain dysfunction, 17 showed mild encephalopathy, and 20 were clinically nonencephalopathic. Severe cases showed obtundation and paratonic rigidity while milder cases showed confusion, inappropriate behavior, inattention, disorientation, and writing errors. There were no focal neurological deficits. The following factors correlated with the severity of brain dysfunction: adult respiratory distress syndrome; fatal outcome; certain types of EEG abnormality; axonal peripheral neuropathy; elevated peripheral white blood cell count; elevated serum levels of alkaline phosphatase, bilirubin, creatinine, phosphate, potassium, and urea; reduced blood pressure and reduced serum albumin level. Our data suggest that brain functions fail with dysfunction of other organs in septic illness. Pathogenetic mechanisms are discussed. The brain dysfunction should be regarded as potentially reversible, even in severely encephalopathic cases. Prompt control of the infection is the most important measure in controlling the encephalopathy and in preventing the increased mortality found with severely encephalopathic patients.