The effect of acetaminophen on fever in bacterial vs. viral infections was tested in 100 children ages 9 days to 17 years who presented to the Pediatric Service with a rectal or oral temperature of 102 degrees F (38.9 degrees C) or greater. All patients were given acetaminophen, 15 mg/kg, and their temperatures were rechecked at 1 hour. Laboratory tests were ordered at the discretion of the examining physician and usually included viral and bacterial cultures and total white blood cell counts. Sixteen patients had proved viral illnesses and 17 patients had serious bacterial infections. There was a significant difference (P less than 0.02) in the white blood cell count between the two groups, with the higher values in patients with bacterial infections. There was, however, no significant difference in the fever response to acetaminophen between the two groups (P = 0.37). The remaining 67 patients were then placed into one of the two groups based on their clinical illness and outcome. The mean temperature change was then calculated between the two groups, and again the difference was found to be statistically insignificant (P = not significant (t = 0.19]. We conclude that there is no correlation between a child's fever response to acetaminophen and the etiology of the fever.