In pediatric patients with community-acquired pneumonia, most of the patients have received antibiotics before admission. In this study, we tried to determine whether we could identify the etiology of pneumonia by clinical and laboratory findings on admission. The etiology of acute pneumonia was studied in 596 pediatric inpatients. A pathogen was identified in 384 (64.4%) episodes of pneumonia. These 384 episodes were divided into six groups as follows; I: pneumonia with blood culture positive or pneumonia with bacterial antigen positive in urine, II: pneumonia with dominant bacterial pathogens in washed sputum. III: Mycoplasma pneumonia, IV: viral pneumonia, V: bacterial (I, II) + viral pneumonia, VI: bacterial (I, II) + Mycoplasma pneumonia. These groups were analyzed by clinical symptoms, physical examination and simple laboratory findings on admission. Patients with Mycoplasma pneumonia have increased blood sedimentation rate, high value of positive C-reactive protein and normal white blood cell count. It was difficult to distinguish bacterial pneumonia from viral pneumonia only based upon clinical symptoms, physical examination and simple laboratory findings.