Hospitals are major risk environments for Legionnaires' disease. Prevention of the disease requires implementation of a systematic program involving active case surveillance in conjunction with the risk assessment in the hospital. Searching the presence of Legionella in the hospital water system is considered as one of the risk assessments in the hospitals. To obtain significant results from the Legionella search process, it is of crucial importance to take samples which best represent the plumbing system. The procedure used for the laboratory analysis of samples also should allow successful isolation of the culprit bacteria. In this article, basic principles of Legionella investigation in the plumbing systems are summarized so as to keep the hospital-associated Legionnaires' disease under control.