All infants aged 29 to 56 days with rectal temperatures in excess of 38.2 degrees C who presented to the Emergency Department of The Children's Hospital of Philadelphia from July 1987 through July 1988 were studied. Each infant was scored (1 to 5) on each of the six items in the Yale Observation Scale by an Emergency Department attending physician before history and physical examination. Individual scores were then added to yield a total score for each patient. An observation score of 10 or less was indicative of a generally well-appearing child, and a score of 16 or more represented an ill-appearing child. Of 126 infants enrolled, 37 (29%) had serious illness; 12 (9.5%) had culture-proven bacterial disease. Of all infants with an observation score less than or equal to 10 (n = 91), 22% had serious illness, and of all infants with an observation score greater than or equal to 16 (n = 20), only 45% had serious illness. The findings suggest that even in experienced hands, the Yale Observation Scale alone does not provide sufficient data to identify serious illness in febrile, 1- to 2-month-old infants.