Anaerobic bacteremia was studied in 32 medical patients (mean age 72 years) in a four-year retrospective analysis. Malignancy was the most common underlying disease and probable portal of entry. The gastrointestinal tract was affected most often, followed by the respiratory and urinary tracts. Bacteremia occurred either following invasive (non surgical) procedures or spontaneously. The clinical course ranged from asymptomatic bacteremia, to mild febrile illness, to sepsis and septic shock (two, 12, 16 and two patients, respectively). The case fatality rate was 25%. The causative organisms were Clostridium and Bacteroides species. All organisms isolated were susceptible to chloramphenicol. Early diagnosis and prompt treatment may reduce mortality in cases of anaerobic sepsis.