Mycoplasma pneumoniae causes not only pneumonia but also other respiratory syndromes such as bronchitis, bronchiolitis, pharyngitis, and croup. These infections mimic viral respiratory syndromes. Most cases are treated on an outpatient basis. Epidemics take place at intervals of 4-7 years. The incidence rate is highest among school children and second highest among children < 5 years of age. Among persons who have had M. pneumoniae pneumonia, rates of subsequent infection with this organism are low, and immunity appears to increase with age. The carrier state may last for several months. Patients with humoral immunodeficiency often develop severe infections due to M. pneumoniae, with involvement of the joints; in these individuals the carrier state may persist. M. pneumoniae has been isolated from bronchial washings from children with AIDS. These children have recovered from mycoplasmal infection with appropriate antibiotic treatment. Dermatologic, neurological, cardiac, renal, and pulmonary complications occur, although data on their frequency are lacking.