Clostridial bacteremia is rare and has a variable presentation from asymptomatic to septic shock with disseminated intravascular coagulation (DIC), red cell hemolysis, and rapid death. In order to delineate the predisposing and prognostic factors in these patients, the authors reviewed 47 cases of clostridial bacteremia presenting over a seven year period at a major metropolitan teaching hospital. Predisposing factors included locally decreased oxidation reduction potential (Eh) in 43 per cent (including atherosclerosis, diabetes, and radiation therapy), systemic immunosuppression in 53 per cent (including alcohol abuse, chemotherapy, steroids, and malignancy), and a site of epithelial barrier disruption. The sites of clostridial invasion included: gastrointestinal tract (GI) (n = 22), pulmonary (n = 7), cutaneous (n = 7), undetermined (n = 7), and female genital tract (n = 4). Seven patients were found to have malignancy. Seventy-nine per cent of the blood culture isolates were histotoxic species (Clostridia perfringens and C. septicum). The overall mortality was 47 per cent. Significant differences between survivors and deaths included DIC, new onset renal failure, severe atherosclerotic disease, and age (P less than .05). The authors conclude that clostridial bacteremia is uncommon but highly lethal and may occur when decreased tissue Eh, systemic immunosuppression, and an epithelial barrier disruption are present. Poor outcome appears to be a reflection of advanced age, underlying illness, and presence of a histotoxic species.