Clinical and microbiologic data of 296 patients with anaerobic bacteremia were reviewed. Anaerobes were isolated with aerobic or facultative bacteremia in 23 instances. The Bacteroides fragilis group accounted for 148 (70%) of 212 isolates of Bacteroides species. B. fragilis accounted for 78% and B. thetaiotaomicron for 14%. Among other species, there were 20 (6%) Fusobacterium organisms, 63 (18%) Clostridium isolates, and 53 (15%) anaerobic cocci. Seventy-five patients died: 40 had B. fragilis group isolates - B. fragilis, 28, and B. thetaiotaomicron, 8 - and 21 had Clostridium organisms isolated. The primary portals of entry were the gastrointestinal tract (42%), decubiti and gangrene (10%), the female genital tract (8%), and the oropharynx (7%). The gastrointestinal tract, decubiti, and gangrene were the predominant sources for B. fragilis and Clostridium organisms, the female genital tract and oropharynx for anaerobic cocci and Fusobacterium species, and the oropharynx for pigmented Bacteroides. Foreign body was associated with Propionibacterium acnes and Clostridium species. Factors predisposing to bacteremia were abscesses, 53; malignancy, 51; surgery, 30; and intestinal obstruction or perforation, 27.