The objective of this study was to determine whether paracetamol (acetaminophen) affects the outcome of children with fever due to bacterial infectious disease. A total of 208 outpatients aged 6 months to 15 years with pyrexia due to bacterial infection who had been examined at the Fujimoto Children's Hospital from March 1992 to May 1992. The number of antipyretic doses of paracetamol (10 mg/kg) a day received within 3 days of illness in the patients with acute fever (> or = 38 degrees C) was investigated. In this study, the patients were divided into two groups: (i) the pneumonia group, which consisted of 101 patients who were subsequently diagnosed as having pneumonia during their illness and (ii) the control group, which consisted of 107 patients who were subsequently diagnosed as having illness with fever that did not progress to pneumonia. The mean number of daily doses was significantly higher for the pneumonia group (2.52 +/- 0.80) than for the control group (1.37 +/- 0.72, P < 0.001). There was no significant difference between the pneumonia group and the control group in body temperature during acute fever (38.7 +/- 0.65 vs 38.8 +/- 0.54 degrees C). The data suggest that frequent administration of antipyretics to children with infectious disease may lead to a worsening of their illness.