To test the association of hyperpyrexia (temperature greater than or equal to 41.1 degrees C) with increased rates of bacteremia and serious bacterial illness in young children, we performed a retrospective case-control study. Seventy-six hyperpyrexic children and an equal number of control cases with temperatures of 39.1-40 degrees C and 40.1-41.0 degrees C were identified. A significantly larger number of diagnostic procedures including blood cultures, urine cultures, chest x-rays and white blood cell counts were performed in the hyperpyrexic children (P less than 0.05). The frequency of serious bacterial infections and bacteremia did not differ among the groups (P greater than 0.05). Hyperpyrexic children need to be evaluated as thoroughly and carefully as any other febrile child but do not merit special consideration.