In 1977, Fraser et al. described an outbreak of pneumonia among legionnaires attending a convention at a hotel in Philadelphia in 1976. Legionnaires' disease (LD) can be nosocomial, community acquired or travel related. The incidence of hospital-acquired legionellosis appears to be increasing. Colonization of water systems by Legionella spp. is ubiquitous in hospitals throughout the world. The outbreak, which later became known as legionnaires' disease, was caused by a new pleomorphic, faintly staining gram-negative bacillus, L. pneumophila, which was isolated at the Center for Disease Control from lung tissues of legionnaires who died. Risk assessment for this disease forms the basis for the institution of control measures. Detection and quantification of Legionella spp. in the environment, in particular in the hospital water distribution system is one of the cornerstones of risk assessment. This review summarizes the current state-of-the-art regarding these aspects and points out important areas which require further study. The environmental surveillance revealed that the centralized hot water distribution system of the hospital was colonized with Legionella. Methods of prevention of the organisms for eradication involved in hospital water systems.