One hundred and seventy-five infants less than 8 weeks of age, presenting to the pediatric emergency room of the Bronx Municipal Hospital Center with rectal temperature greater than or equal to 38 degrees C (100.4 degrees F), were studied. House officers recorded their impressions of the infants on a number of variables prior to performing a lumbar puncture and obtaining laboratory data. All infants were admitted for parenteral antibiotic therapy pending culture results. Culture-positive bacterial infections occurred in 6.3% (n = 11); the incidence of bacteremia was 3.4% (n = 6). Of special concern were the 134 infants who had no visible source for their fever during the first examination. A major goal was to determine whether there were any early predictors of bacteremia in this group. The individual variables of white blood cell count greater than or equal to 15,000/mm3, band count greater than or equal to 500/mm3, temperature, impression of irritability, tone, cry, and activity level were not related to the presence of bacteremia. An erythrocyte sedimentation rate greater than or equal to 30 and the examiner's impression of sepsis were significantly associated with bacteremia but did not correctly identify all cases. However, the combination of impression of sepsis, white blood count greater than or equal to 15,000/mm3, and erythrocyte sedimentation rate greater than or equal to 30 identified all infants with bacteremia and excluded 82% of the infants who were eventually shown not to have bacteremia.