A prospective pilot study of new episodes of fever in patients hospitalized in a children's hospital revealed 51 episodes over a ten-week period. More than half of these were apparently hospital-acquired respiratory infections, with group A streptococci or parainfluenza virus type 3 the most frequent agents recovered. About one fourth of the episodes represented brief episodes of fever following operative procedures, without evidence of any infection. The remainder of the episodes were complications or recurrences of the patient's basic disease, or were of unknown cause.