Standard references continue to recommend testing specimens processed on the BACTEC TB System for 8 weeks, despite evidence that mycobacteria are rarely recovered beyond 5-6 weeks. To clarify this issue, we retrospectively reviewed all positive blood/bone marrow cultures processed during a 17-month period when specimens were tested for 6 weeks. We then prolonged the incubation period to 8 weeks during the subsequent 5 months. Excluding Mycobacterium genavense, only 1 of 159 mycobacterial isolates was recovered during and none were recovered beyond the 5th week of incubation and testing on the BACTEC TB System.