Objective. Using a continuously monitoring blood culture system, we determined the time to positivity of blood cultures performed on immunocompetent infants and children who were not receiving antibiotics at the time of culture. Study Design. This study was conducted prospectively using blood cultures taken in the emergency department and outpatient clinics of an urban pediatric teaching hospital from February 1, 1993, through December 31, 1996. Cultures were excluded if obtained from patients receiving antibiotics, patients with a central line, patients with prosthetic devices, or those being followed by the oncology division. Our measures included: 1) recording the time to positive culture obtained by using a continuously monitoring blood culture instrument, 2) patient information derived from the hospital computer system concerning antibiotic use and the presence of indwelling central venous catheters and prosthetic devices, and 3) a chart review of 10% of patients from whom positive cultures were obtained. Results. During the 47-month study period, 10 200 single bottle blood cultures were obtained, 711 (6.97%) of which became positive. Patients ranged in age from <1 week to 24 years (mean: 2.00 years). Two hundred fifty-eight cultures (36.3%) contained only pathogens, 370 (52%) contained only skin contaminants, and 83 (11.7%) contained a mixture of contaminant and pathogen. Of the 258 cultures containing only pathogens, 14% were positive by 12 hours, 87% by 24 hours, 92% by 36 hours, 95% by 48 hours, 98% by 60 hours, and 99.7% by 72 hours. Ninety-five percent of critical pediatric pathogens including Streptococcus pneumoniae, Salmonella and other Enterobacteriaceae, Neisseria meningitidis, and groups A and B streptococci were detected in <24 hours. Conclusion. Because 87% of all cultures containing pathogens were detected within the first 24 hours of incubation, this study can assist emergency department, clinic, and primary care clinicians when making critical decisions concerning patients on whom blood cultures were obtained. Data on time to positivity of blood cultures can be used in conjunction with clinical status to support clinicians in making patient management decisions. Use of short stay (≤24 hours) or extended care units requiring less patient supervision may be easier to justify when a continuously monitoring blood culture instrument is used in the microbiology laboratory.