Mycoplasma pneumioniae has a unique genomic composition, cellular biology, and a fastidious nature as the smallest cell-free living organism that lacks a cell wall. Previous studies have suggested that a clinical manifestation of a M. pneumoniae infection is a consequence of a host immune response, particularly involving cellular immunity. Adenosine deaminase (ADA) is the main T-lymphocyte enzyme, and its activity is high in diseases where cellular immunity is stimulated. Therefore, its activity is useful for diagnosing a tuberculous pleural effusion. A pleural effusion is found in 5-20% of Mycoplasma pneumonia patients. However, there are few reports of high ADA activity in a mycoplasmal pleural effusion. Here we report a case of Mycoplasma pneumoniae infection established by a polymerase chain reaction and serologic tests, accompanying high ADA activity in a pleural effusion.