The humidification trays of five of seven incubators in a neonatology unit of a hospital were found to be colonized with Legionella pneumophila, serogroup 1. Bacteriological analysis of the water in the humidification trays showed very large numbers of heterotrophic bacteria, one of which also contained Pseudomonas aeruginosa. Two hot water systems supply the neonatology unit, either of which is used to add water to the humidification trays; one system (A) is maintained at about 60 degrees C, while the other system (B) is maintained at 45 degrees C. The latter was also found to be colonized with L. pneumophila, Sg1. Monoclonal antibody (Mab) subgrouping of the isolates, indicated that system B was the source of colonization of the humidification trays of the incubators.