Mycoplasma pneumoniae (M. Pneumoniae) is a primarily pathogen of the respiratory tract. The clinical characteristics, laboratory findings, and roentgenographic patterns of 45 patients with serologically proven M. pneumoniae pneumonia admitted to Chang Gung Hospital from 1981 to 1989 have been reviewed. There were 23 males and 22 females. Forty-one (91%) were below 40 years old and 13 patients (29%) were below 5 years old. Fever, cough and chest rales were the most common symptoms and signs. A transient mild elevation of liver enzymes was seen in 33% of the patients, most of whom were below the age of ten (73%). A leukocyte count over 15,000/cu mm was not rare (16%). Roentgenographic features included unilateral infiltration (84%), lower lobe predominance (60%), and either confluent (56%) or patchy (33%) consolidation. Pleural effusion occurred in 24% of the patients. Complete resolution of chest roentgenography took from 8 to 42 days with a mean of 20 days. The response of fever to treatment with erythromycin took from 1 to 6 days with a mean of 3 days. There were no life threatening pulmonic or extrapulmonic complications.