Abstract We performed a controlled evaluation of ultraviolet light irradiation of hot and cold water supplies in a hospital colonized with Legionella by inserting the u.v. units near the “point-of-use”. Showers on one wing of a hospital were supplied with u.v.-irradiated water. Showers on another wing and 20 other outlets served as controls. Cultures prior to the installation of u.v. units showed all outlets to be heavily colonized with L. pneumophila. Despite disinfection of incoming water by u.v., Legionella in established niches of the plumbing system continued to survive. Superheat/flush and chlorination was then applied prior to u.v. activation. Although cultures became negative initially, recolonization occurred after I month. Scale accumulation on u.v. lamps had compromised the efficacy of u.v. irradiation. Filters were added to prevent scale accumulation on the u.v. lamps and the u.v.-supplied showers remained Legionella-free. U.V. plus prefiltration can prevent Legionella recolonization for at least 4 months after disinfection if the u.v. units are installed near the “point-of-use”. Disinfection must be directed not only at incoming water, but also at established niches within the existing plumbing system. The advantages of u.v. include easy installation, low expense, and no adverse effects on water or plumbing.