Mycoplasma pneumoniae and the TWAR agent account for a good proportion of pneumonias acquired in the community among older children and young adults. Recovery from these infections is common, although serious complications may occur. Each is associated with fever, nonproductive cough, and headache. The full clinical manifestations of TWAR agent infection are only now beginning to be defined. Diagnosis of M pneumoniae can be by serology and/or culture capabilities. The TWAR agent cannot be routinely isolated and serologic tests are available only in research laboratories. Response to tetracycline antibiotics has been established for each agent and hospitalization is rarely needed. Erythromycin and other macrolide antibiotics have good activity against M pneumoniae and can be employed as an alternative to tetracycline antibiotics for this pathogen.