Four cases of legionnaires' disease were diagnosed by specific serologic tests in a group of 33 immunocompromised patients admitted to the same hematologic department for acute febrile pneumonitis. The underlying disease of these four patients was hairy-cell leukemia (HCL) in three cases and allogeneic bone marrow transplantation in the other. This article stresses the enhanced susceptibility of patients with HCL to Legionella pneumophila and discusses its possible causes, especially monocyte deficiency. We propose the use of erythromycin as part of the initial empiric antibiotic therapy in immunocompromised hosts with acute pneumonitis until the results of specific serologic tests or isolation of L pneumophila is obtained.