A controlled evaluation was made of UV irradiation in a hospital colonized with legionella by inserting the UV units near the "point-of-use." Showers on one wing were supplied with UV-irradicated water. Showers on another wing served as controls. Efficacy was evaluated in stepwise fashion: (1) UV installation only without prior decontamination. Legionella in established niches survived despite UV disinfection documenting the important of Legionella colonization within existing biofilms. (2) UV installation after decontamination by superheat and chlorination. Although cultures became negative initially, recolonization occurred after 1 month. Scale accumulation on UV lamps compromised UV irradiation. (3) UV + filtration after superheat/flush and chlorination. With addition of filters, the UV-supplied showers remained Legionella-free for at least four months. (4) Filtration alone after superheat/flush and chlorination. Legionella recolonized withine one month, showing that the effect of filtration was prevention of scale rather than filtering out Legionella. The advantages of UV include easy installation, low expense, and no adverse effects on water or plumbing.