The number of positive anaerobic blood culture results per 1,000 blood cultures performed decreased from 12.6 in the period from 1997 to 2001 to 7.0 in the period from 2002 to 2006 (P < 0.001), as did the proportion of isolated anaerobic organisms compared to the number of all organisms isolated from blood cultures (7.6% to 4.3% [P < 0.001]), while positive aerobic cultures remained stable. In contrast, the proportion of Bacteroides fragilis group members and gram-positive cocci within the anaerobic group increased (26.8% to 36.7% [P = 0.004] and 5.4% to 12% [P < 0.001], respectively). The number of patients with anaerobic bacteremia decreased from 122 patients in 1997 to 69 in 2006.