Cytomegalovirus (CMV) pneumonitis is a common complication of AIDS. With the development of potentially effective drug therapy, the indications for treatment and best means of diagnosis have become issues of practical importance. We reviewed the clinical records and autopsy material from 54 patients dying with AIDS, 39 (72 percent) of whom had CMV infection documented postmortem by culture and the presence of inclusion bodies. Of the patients with CMV infection, disseminated disease was documented in 23 (59 percent) and pneumonitis in 31 (80 percent). Although the majority of patients with CMV pneumonitis had additional forms of pulmonary pathology, CMV was the only causative agent identified in two patients with severe pulmonary disease. Since CMV can cause severe lung damage and disseminated infection in patients with AIDS, specific treatment may be beneficial. Improved diagnostic technology is needed in this area.