The relative value of routine anaerobic blood culture for recovery of organisms and identification of episodes of bloodstream infection was assessed in a three-component, high-volume blood culture system which employs aerobic and anaerobic bottles of BacT/Alert (Organon-Teknika, Durham, N.C.) and aerobic cultures of Isolator (Wampole Laboratories, Cranbury, N.J.). The results of 5,595 blood culture sets from patients with suspected bloodstream infection were analyzed. Compared with either the aerobic BacT/Alert bottle or aerobic culture of Isolator, the BacT/Alert anaerobic bottle recovered significantly fewer isolates (242 versus 294, P < 0.05; 242 versus 298, P < 0.05) but did not detect significantly fewer episodes of bloodstream infection (141 versus 157, P > 0.05; 141 versus 147, P > 0.05). The BacT/Alert anaerobic bottle recovered significantly more isolates of obligately anaerobic bacteria (16 versus 4, P < 0.05; 16 versus 0, P < 0.05) and detected significantly more episodes of bloodstream infection caused by obligately anaerobic bacteria (10 versus 3, P < 0.05; 10 versus 0, P < 0.05) than either the aerobic bottle of BacT/Alert or the aerobic culture of Isolator. The combination of the BacT/Alert anaerobic bottle and the aerobic culture of Isolator recovered as may isolates (374 versus 377) and detected as many episodes of bloodstream infection (194 versus 191) as the combination of the aerobic bottle of BacT/Alert and the aerobic culture of Isolator, and both of these combinations identified at least 8% more isolates and detected at least 3% more bloodstream infections than the combination of the BacT/Alert aerobic and anaerobic bottles. Further analysis of the data revealed that the utility of the BacT/Alert anaerobic bottle, especially when combined with the aerobic culture of Isolator, resulted from not only enhanced recovery of obligately anaerobic bacteria but also effective recovery of facultatively anaerobic bacteria. These results demonstrate the utility of the anaerobic BacT/Alert bottle for detecting bloodstream infection caused by either facultatively anaerobic bacteria or obligately anaerobic bacteria and support the routine inclusion of anaerobic blood culture in the three-component blood culture system used in our hospital.