To evaluate the use of routine anaerobic blood cultures with the BacT/Alert system, results of 12,289 blood culture sets collected from adults over a 9-month period were reviewed. Of the sets included in the study, 1,306 (10.6%) from 808 patients grew 1 or more organisms. Anaerobes were present in 39 sets from 38 patients. Of the positive sets, both bottles were positive in 60.7% of cases, the aerobic bottle only in 23.7%, and the anaerobic bottle only in 15.6%. When only the 254 patients who had 2 or more positive sets were considered, both bottles were positive in 71.5% of cases, the aerobic bottle only in 20.7%, and the anaerobic only in 7.8%. In this subset of patients gram-positive bacilli, gram-negative bacilli other than Enterobacteriaceae, and yeasts grew significantly more frequently in aerobic bottle. No organisms preferred the anaerobic bottle. These data support the selective use of the BacT/Alert anaerobic blood culture bottle in patients at risk for anaerobic bacteremia.