To determine if observational assessment performed in a systematic manner adds to the efficacy of the traditional history and physical examination in detecting serious illnesses in febrile children, and to determine the sensitivity of the combined evaluation, we studied consecutive patients less than or equal to 24 months of age seen for evaluation of fever at the Primary Care Center-Emergency Room (PCC-ER) of the Yale-New Haven Hospital (n = 143) and a suburban private practice (n = 207). An attending pediatrician performed the observation using the previously reported Acute Illness Observation Scales (AIOS). Subsequently, the history and physical examination were done by an attending pediatrician, and findings were scored as to whether they suggested the presence of a serious illness. Thirty-six patients, 28 in the PCC-ER and eight in the private practice, had a serious illness. The combined AIOS, history, and physical examination had a higher sensitivity and r correlation for serious illness than did the traditional history and physical examination. Three children with serious illnesses, all of whom had no abnormalities on history and physical examination, were identified only by use of AIOS. We conclude that assessment of appearance in a child with fever, when performed in a careful, integrated, stepwise fashion, improves the sensitivity of the history and physical examination in detecting serious illnesses in febrile children.