The performance of the Baby Check Score System for grading severity of acute systemic illness in infants under 6 months of age was assessed prospectively in 394 of the 495 babies who were brought to the Emergency Room of Veterans General Hospital (VGH), Taipei from April 1, 1992 to July 23, 1992. The scores were given by the interns. Duty residents graded the severity of each baby's illness without information on the score. Babies with serious illness scored higher than those with minor illness. The median score for well babies was 2 (range 0-5), for mild illness was 7 (range 0-20), for moderate illness was 14 (range 0-37), and for serious illness was 26 (range 12-40). Correlation between the assigned score and the grading of duty residents was good. The score's false negative rate to underestimate the moderately or seriously ill babies was 14.2% and false positive rate to overestimate well or mildly ill babies was 10.7%. Appropriate use of the Baby Check Score in the Emergency Room can help junior doctors detect serious illness, and reduce unnecessary admissions for only minor illness.