We reviewed 402 hospital admissions of patients with acute leukemia to define the frequency and characteristics of anaerobic bacteremia in this patient population. Six (5.2%) of the 116 septicemia episodes documented in these patients were caused by anaerobes (Bacteroides species, 3; Fusobacterium species, 2; and Clostridium tertium, 1); two of these episodes were polymicrobial. Five patients had had prior bacteremia. All six patients were receiving broad-spectrum antibiotics, including an anti-pseudomonal penicillin, at the time of the episode. In each instance, the absolute granulocyte count was 0/mm3. Five patients had clinically apparent sources of infection, including perirectal abscess, gastrointestinal bleeding, or Clostridium difficile-associated diarrhea. Anaerobic bacteremia is an infrequent occurrence in granulocytopenic patients with acute leukemia, but may occur when there is obvious disruption of normal gastrointestinal anatomic barriers.