A cohort of 484 febrile children were examined to (1) assess the utility of temperature response to acetaminophen as a diagnostic test for occult bacteremia (OB) and (2) compare it with the white blood cell (WBC) count. For a period of 18 months, the records of all febrile children seen in the emergency department were reviewed. Testing a response to acetaminophen of a < or = 0.8 degrees C decrease in temperature, the sensitivity, specificity, and positive and negative predictive values were 47%, 74%, 12%, and 95%, respectively. When compared with a WBC count > or = 15,000/microL, the corresponding values were 76%, 65%, 15%, and 97%. Receiveroperator characteristic curves were constructed to compare temperature responses with WBC values. The difference was not significant (P > 0.05). The conclusion reached was that temperature response to acetaminophen has predictive values that are similar to the WBC count, and it may provide useful information. However, neither test is impressive, and the clinician cannot reliably predict which febrile children are at risk.