Legionella pneumonia is frequently reported in immunocompromised patients. The most important predisposing factor is a quantitative and qualitative defect of the monocytic-macrophagic system. We report a case of Legionella pneumophila in a 43-year-old man revealing hairy cell leukemia. The patient presented with community-acquired pneumonia resistant to common antibiotics. Serology provided the diagnosis and the patient improved with adequate treatment. Early diagnosis and appropriate treatment may reduce the mortality of this serious complication.