The case records of 258 children with adenovirus; influenza A or B virus; parainfluenza 1, 2, or 3 virus; or respiratory syncytial virus infections were studied retrospectively with special attention to the degree and duration of fever. A temperature of 39.0 degrees C or higher was most frequently recorded in adenovirus, influenza A, and influenza B virus infections (in 68%, 84%, and 65%, respectively). The mean highest degree of fever in respiratory virus infections (39.2 degrees C +/- 0.6 degrees C) during hospitalization did not differ from that in defined serious bacterial infections, ie, meningitis, epiglottitis, sepsis, and urinary tract infections (39.3 degrees C +/- 0.7 degrees C). The mean duration of fever varied from 2.5 days (parainfluenza 2) to 5.2 days (influenza B). Of all children with respiratory virus infections, 37% had fever lasting five days or longer. The data show that high and prolonged fever is frequently associated with respiratory virus infections in hospitalized children and that it does not differ significantly from fever in severe bacterial infections.