Background : Mycoplasma pneumonia (M. pneumonia) is a major cause of atypical pneumonia, and its incidence is predominantly at childhood and early adulthood. In contrast, the incidence of adult patients with M. pneumonia has been known to be low. Furthermore the clinical aspects of M. pneumonia are different from those of community acquired pneumonia. Thus, we evaluated the clinical aspects of M. pneumonia in the adult patients. Method : Mycoplasma antibody and cold agglutination tests were performed in patients with clinically suspected pneumonia who had abnormal infiltrations on chest P-A. The 12 patients with pneumonia, who fulfilled entry criteria of more than 1:64 of cold agglutination titer and 1:40 of mycoplasma antibody titer or four-fold increase of mycoplasma antibody titer during one week, were analyzed in terms of clinical aspects. Results : 1) Twelve patients, male 3 and female 9, were included in this study. The peak incidence was teenager. 2) M. pneumonia occured perennially, but predominantly between June to October in eight patients. 3) The main symptoms were fever, coughing, sputum. 4) The main patterns of chest P-A were bronchopneumoina in 8 cases, and involved lesion were nearly both lower lobe. Conclusion : The clinical aspects with Mycoplasma pneumonia in adult patients were different from those of community acquired pneumonia.