The results of early studies showed that anaerobes accounted for 20% of all bacteremias; more-recent data suggest that these organisms account for ∼4% (0.5%–9%) of bacteremias (or approximately one case per 1,000 admissions), with variation by geographic location, hospital patient demographics, and especially, patient age. Elderly persons seem to be at increased risk for developing anaerobic bacteremia while young children (2–5 years of age) are at the least risk. Bacteroides fragilis is the most common blood isolate recovered from patients with anaerobic bacteremia; this organism and species of the B. fragilis group account for ∼55% of anaerobic bacteremias. B. fragilis bacteremia is associated with a mortality of 19%, with a mortality risk of 3.2; a 16-day increase in hospital stay; and often, intra-abdominal disease. Associated risks for mortality include chronic liver disease and congestive heart failure. There is value in performing separate anaerobic blood cultures; clinicians at each institution should determine the prevalence of anaerobic bacteremia and use this information to guide blood-culture practices.