The initial chest radiographs of 31 patients with laboratory-proved pneumonia were evaluated by a panel of 6 radiologists who had no prior knowledge of the clinical data. No statistical reliability was found for distinguishing bacterial from nonbacterial pneumonia. Radiographic diagnoses were 67% accurate for the 16 cases of bacterial pneumonia, and 65% accurate for the 9 viral cases. Six cases of Mycoplasma pneumonia were diagnosed as nonbacterial 19% of the time, and as "bacterial pneumonia" 81% of the time.