To evaluate whether the use of BacT/Alert FA and FN blood culture bottles increases the yield of microorganisms, we performed a before-after study. BacT/Alert standard aerobic (AE) and anaerobic (AN) bottles were used from January 1999 to May 2001 (before period). FA and FN bottles were used from May 2001 to March 2003 (after period). A total of 7796 AE, 7807 AN, 4798 FA, and 4787 FN bottles were processed. There were 742 (9.5%) AE-, 598 (7.7%) AN-, 521 (10.7%) FA-, and 396 (8.3%) FN-positive bottles. From these positive bottles 776, 631, 585, and 487 microorganisms were isolated, respectively. Among the isolated microorganisms, 58 (7.5%) and 59 (10.1%) Candida species were isolated from AE and FA bottles, respectively, and 17 (2.7%) and 21 (4.3%) obligate anaerobes were isolated from AN and FN bottles, respectively. We conclude that BacT/Alert FA and FN bottles showed a higher percentage of positivity for microorganisms, in particular for Candida species and obligate anaerobes.