Legionella pneumophila is an important cause of severe community-acquired pneumonia. An unusually high frequency of L. pneumophila pneumonia has been described among patients with hairy cell leukemia; however, L. pneumophila is still underestimated and frequently remains undiagnosed among patients with this malignant disease. We describe the case of a middle-aged man who developed severe sepsis and multiorgan failure owing to L. pneumophila pneumonia in the setting of newly diagnosed hairy cell leukemia. © 2011 by Lippincott Williams & Wilkins.