Legionnaires' disease is a rare cause of community acquired pneumonia in children and is exceedingly rare in infants and neonates. We describe a case of a 15 years old female, with lymphoblastic leukemia and legionella pneumophila pneumonia. Diagnosis was made by specific culture and polymerase chain reaction method from sputum and bronchoalveolar ravage specimens. Treatment was prolonged because of slow resolution and the fact that she underwent bone marrow transplantation. This emphasizes the importance of considering this rare entity in cases of severe pneumonia, especially in the immunocompromised host.