Objective: To determine whether environmental cultures for Legionella increase the index of suspicion for legionnaires' disease (LD). Design: Five-year prospective study. Setting: Twenty hospitals in Catalonia, Spain. Methods: From 1994 to 1996, the potable water systems of 20 hospitals in Catalonia were tested for Legionella, Cases of hospital-acquired LD and availability of an "in-house" Legionella test in the previous 4 years were assessed. After the hospitals were informed of the results of their water cultures, a prospective 5-year-study was conducted focusing on the detection of new cases of nosocomial legionellosis and the availability and use of Legionella testing. Results: Before environmental cultures were started, only one hospital had conducted active surveillance of hospital-acquired pneumonia and used Legionella tests including Legionella urinary antigen in all pneumonia cases. Only one other hospital had used the latter test at all. In six hospitals, Legionella tests had been completely unavailable. Cases of nosocomial LD had been diagnosed in the previous 4 years in only two hospitals. During prospective surveillance, 12 hospitals (60%) used Legionella urinary antigen testing in house and 11 (55%) found cases of nosocomial legionellosis, representing 64.7% (11 of 17) of those with positive water cultures. Hospitals with negative water cultures did not find nosocomial LD. Conclusions: The environmental study increased the index of suspicion for nosocomial LD. The number of cases of nosocomial LD increased significantly during the prospective follow-up period, and most hospitals began using the Legionella urinary antigen test in their laboratories.