We have examined 221 cases of mycoplasmal pneumonia in adults during the past 17 years. During this time epidemic waves occurred every three to four years. The incidence of disease was highest in patients 20 to 30 years of age. The most common clinical features were cough, fever, sputum, and rales. The most characteristic feature was a persistent cough which lasted about three to four weeks. Roentgenographic examinations showed a variety of patterns, but the most consistent feature was a feathery shadow, appearing in the lower field of either or both of the lungs. Lung function tests showed peripheral airway impairment. Although roentgenographic examination provided useful information, it could not be used as a pathognomic feature of mycoplasmal pneumonia. Tetracyclines were most effective for eliminating clinical symptoms, whereas the macrolides provided the best response based on roentgenographic evaluations.