Patients will be recruited in the ED and pediatric ward at Assaf Harofeh Medical Center. Legal guardians of eligible patients will be approached for consent. After obtaining informed consent a detailed history regarding the way fever was managed during the current illness will be collected (appendix 1). Patients will be randomized in a ratio of 2:1 (by a computer generated list of random numbers) into one of two groups. Group A will be treated with oral suspension of ibuprofen 10mg/kg (if the patient received acetaminophen at home) or oral suspension of acetaminophen 15 mg/kg (if treated previously with ibuprofen). The second group will be treated with the same amount of oral placebo suspension. The parents and the physician will be blinded to the treatment given. Body departure will be recoded at base line and at 15, 30, 60, 90, 120, 180 minutes after the drug administration. Blood sample for serum concentrations of acetaminophen or/and ibuprofen will be taken at the time of routine blood sampling according to the attending physician decision. Serum concentrations will not be measured if the attending physician decides that blood tests are not indicated.