In a prospective study, 330 consecutive children less than 24 months old coming to the emergency room of Yale-New Haven Hospital with a temperature greater than or equal to 40 C were evaluated. Nearly all patients had a white blood cell (WBC) count, erythrocyte sedimentation rate (ESR) (Wintrobe), blood culture, and chest roentgenogram. Eighty-eight percent were evaluated 24 to 48 hours later. The mean WBC count and ESR were significantly elevated in children with positive blood cultures or pneumonia. The risk of bacteremia was increased threefold and the risk of pneumonia was increased twofold in children with a WBC count greater than or equal to 15,000/cu mm or an ESR greater than or equal to 30 mm/hr compared to children without leukocytosis or elevated ESR. Sixty-one percent of children with bacteremia or pneumonia. 63% of children in whom these diagnoses were not apparent on physical examination, and 86% of children with otitis media complicated by pneumonia or bacteremia had either a WBC count greater than or equal to 15,000/cu mm or an ESR greater than or equal to 30 mm/hr. A WBC count greater than or equal to 15,000/cu mm and an ESR greater than or equal to 30 mm/hr were more effective than a polymorphonuclear leukocyte count greater than or equal to 10,000/cu mm and/or a band count greater than or equal to 500/cu mm in screening young children with high fever for bacteremia, pneumonia, or complicated otitis media.