THE ETIOLOGIC agent of most cold-agglutininpositive primary atypical pneumonias has been identified1asMycoplasma pneumoniae. The roentgenographic findings are similar to those of bronchopneumonia2with an enlarged hilar shadow and exaggerated bronchial markings or patchy consolidation.3In approximately 50% of primary atypical pneumonia cases, the lesion is limited to a Single lobe4; however, these infiltrates rarely involve the entire lobe. True lobar or segmental pneumonia, which involves an entire lobe or segment, is usually caused by the pneumococcus organism,Diplococcus pneumoniae,5and is uncommonly seen in primary atypical pneumonia. This report describes an unusual case ofM pneumoniaepneumonia which presented as lobar pneumonia, progressed to involve the entire right lung, and produced pleural effusion. Report of a Case A 13-year-old girl was admitted to the Children's Hospital of Pittsburgh with a five-day history of fever and a nonproductive cough. The fever was intermittent with