A nosocomial pneumonia caused by Legionella pneumophilaserogroup 2oc–14oc curred in a 7-year-old patient following allogeneic hematopoietic stem cell transplantation for thalassemia major. The patient was diagnosed as nosocomial Legionella pneumonia by a polymerase chain reaction (PCR) from bronchoalveolar lavage and a culture of L. pneumophilaserogroup 2–14 from patient’s room faucet water. Legionella was eradicated from our hospital’s water distribution system by superheating and chemical eradication methods (hyper-chlorination and hydrogen peroxide). We did not detect any case after this event. Early recognition of contamination of hospital water system with Leigonella proves the importance of prevention new cases.