The specimen capacity of blood culture systems is determined by the length of time that blood cultures are incubated. Since the patient populations served by hospitals vary, individual laboratories should evaluate the relative cost and benefits of different testing regimens for their particular setting. To be cost-effective, the use of a 5-day rather than a 7-day protocol for the Bactec 9240 system (Becton Dickinson Diagnostic Instrument Systems, USA) has been recommended. To evaluate whether the shorter schedule would be appropriate at the Microbiology Laboratory at the National Cheng Kung University Medical Center in Tainan, Taiwan, the yield from 5 days versus 7 days of incubation was compared using a total of 9653 blood specimens collected from 1 April to 30 September 1997. Of the 1848 positive vials, 1822 (98.6%) were positive in the first 5 testing days; only 26 (1.4%) were positive on day 6 or 7. Only five of the latter vials were judged to contain clinically significant organisms: one Cryptococcus neoformans, one Candida albicans, one Enterobacter cloacae, one Klebsiella pneumoniae, and one Proteus mirabilis. Two vials contained organisms whose clinical significance was categorized as unknown, while 19 contained organisms that were considered contaminants. These data suggest that, for the laboratory studied, the 5-day protocol for the Bactec 9240 system is more cost-effective than the 7-day protocol and is adequate for detection of positive blood cultures.