Data regarding parapneumonic pleural effusion in M. pneumoniae pneumonia (MP) patients are limited. The purpose of this study was to examine the characteristic of pleural fluid in patients with parapneumonic effusion caused by M. pneumoniae and to investigate the clinical features of MP patients with pleural effusion. Thirty-two consecutive MP patients were collected. Of the 32 patients, three underwent thoracentesis and pleural fluid analysis. Next, we searched previous medical records and found three additional patients whose data of pleural fluid analysis were available. MP patients with pleural effusion tended to be younger and had longer hospital stays and more common use of systemic steroids compared with those without pleural effusion. In five of the six patients whose pleural fluid data were available, the pleural effusion was lymphocyte-predominant rather than polymorphonuclear leukocyte-predominant; these patients also had elevated adenosine deaminase levels. Taken together, these results indicate that MP patients with pleural effusion may have a more severe form compared with those without pleural effusion. M. pneumoniae should be considered an etiologic agent for lymphocyte-predominant pleural effusion.