We evaluated the efficacy of the ultraviolet irradiation on hospital water colonized by Legionella pneumophila serogroup 3, by inserting a lamp system on a hot water pipe supplying a small area. Cultures were performed for four months from 5L samples of water, collected before and after the ultraviolet treatment at the lamp unit and from two distal points. Irradiation was effective immediately after disinfection (<10 cfu/L), even when the incoming water was highly contaminated. One distal point showed little or no contamination (<10-20 cfu/L), while the other showed little to moderate contamination (<10(3) cfu/L). We conclude that ultraviolet irradiation is useful to protect the water system in small area; however, because of the lack of residual activity, it should be combined with other methods of disinfection. Maintenance of the water system is also necessary in order to reduce biofilm formation and Legionella recolonization.