Young, febrile children are at risk for occult bacteremia. However, some positive blood cultures are contaminants. This study assesses the ability of preliminary blood culture results to distinguish between pathogens and contaminants. Positive blood cultures were obtained and evaluated from 210 otherwise well febrile children, at risk for occult bacteremia, three months to three years of age, who were without invasive bacterial disease. These children had entered a pediatric emergency department between 1983 and 1989 and were not admitted to the hospital. Blood cultures of true pathogens became positive more rapidly than contaminants, 13.8 +/- 7.0 hours versus 37.6 +/- 29.9 hours, respectively (P < 0.0001). Gram stain results were used as a diagnostic test for pathogenicity with a sensitivity of 98.1% and specificity of 54.5% for the entire population, and 100% and 77.3% respectively, for those cultures becoming positive after 24 hours. Time to positivity and initial gram stain results are valuable diagnostic tests in distinguishing between pathogens and contaminants for blood cultures obtained from children at risk for occult bacteremia.