Pneumonia caused by Legionnaires' disease bacterium was recognized in eight patients during a 7-month period. The patients were immunosuppressed by their underlying illness, corticosteroid therapy, and other exogenous immunosuppressive agents. Five of the patients had received immunosuppressive therapy for less than 16 days. Clinical presentation was similar to that of other bacterial pneumonias in compromised patients. Legionnaires' disease progressed to necrotizing pneumonia with abscess formation and respiratory failure in two patients. Diagnosis was made by [1] culture of lung tissue and bronchial washings; [2] direct fluorescent antibody staining of lung tissue, sputum, and bronchial washings; and [3] serologic evidence of infection. Therapy with oral erythromycin was ineffective. Intravenous erythromycin was given to six patients, with a good response. However, two patients showed further clinical improvement after rifampin was added. Because this illness may be more severe in compromised hosts, open lung biopsy and special microbiologic tests should be done when Legionnaires' disease is suspected.