The antipyretic effect of single doses of naproxen, 2.5 mg/kg or 7.5 mg/kg, was compared to that of aspirin, 15 mg/kg, and of a placebo in a double-blind study. Febrile children, hospitalized overnight, were randomly assigned to one of the treatment or placebo groups, and their temperatures were recorded hourly. Naproxen, 7.5 mg/kg, was at least as effective as aspirin in onset of action and reduction of fever, and had a longer duration of action. Each of the three treatments had a significant effect as compared to that of the placebo. There was no difference in the total incidence of side effects among the four groups, but mild gastrointestinal complaints were more common among the patients in the high-dose naproxen group, whereas CNS complaints were more prevalent among the patients in the placebo group.