Using indirect immunofluorescence, the authors examined 713 serum specimens submitted for Pneumocystis carinii serologic studies from 566 patients for antibody to the Legionnaires' disease bacterium. This group was chosen because it presumably consisted largely of immunosuppressed patients with acute respiratory illnesses. Of patients tested, 3.4% had titers of 1:128 or greater to Legionella pneumophila. Four (3.7%) of 107 patients for whom multiple specimens were submitted showed diagnostic increases in titer. The proportion of seropositive specimens did not vary with the age, sex, or geographic location of the patients or with season of the year in which the specimens were submitted. In a separate group of 138 serum specimens from 48 patients undergoing marrow transplantation, only 1 seropositive specimen was detected. No estimate of incidence is possible from these studies, but serologic evidence of past or current infection with Legionella pneumophila is uncommon in patients in whom Pneumocystis carinii pneumonia is suspected on clinical grounds. Nevertheless, Legionnaires' disease can affect the immunosuppressed host and should be considered in the differential diagnosis of pneumonia in such a patient.