During the 32 months between October 1984 and May 1987 there were 72,800 admissions in our center; 31,523 blood cultures were undertaken and in 3,831 of them (11.5%) at least one significant organism was recovered; 6.7% of blood cultures yielded anaerobic bacteria, and 3.1% Bacteroides spp. In the 1,478 instances of bacteremia recorded during that period, the relative frequencies of bacteremia by anaerobic bacteria and by Bacteroides were 8.6% and 3.9%, respectively. In overall 50 Bacteroides bacteremias (47 due to Bacteroides fragilis and 3 to Bacteroides spp), 32 (64%) were hospital acquired and 14 (28%) postsurgical. Their most common source was the abdominal cavity (25, 50%), followed by the skin and soft tissues (7, 14%), feminine pelvis (4, 8%), miscellaneous (8, 16%), and unknown (7, 14%). Fifteen bacteremias (30%) were polymicrobial, 20 (40%) were associated with septic shock, and 15 (30%) with renal failure. Overall mortality rate was 52%, and that directly attributable to bacteremia was 32%. Poor prognostic predictors included the source from a perforated viscus, septic shock, renal failure or gastrointestinal hemorrhage, and the absence of adequate antimicrobial therapy.