Hitherto, nosocomial epidemic outbreaks of legionnaire's disease have not been reported in Denmark. Only sporadic cases have been described. A survey of 75 Danish hospitals concerning the dimensions and operation of the hot-water systems revealed that only 13% had a hot water tank temperature above the recommended 60 degrees C. Temperatures of coldest tap water and of returning water were above 50 degrees C in only 31% and 24% respectively. Twelve representative hospitals with 35 independent water systems were investigated for the presence of Legionella pneumophila (Lp). Lp was demonstrated in all the hospitals and in 34 (97%) of the systems. The prevailing serogroups were (in order of frequency) 3, 1, 2, 5 and 6. We conclude that the operation of the hot tap water systems should be optimized. This report indicates that nosocomial epidemics are conceivable and attention should be paid to the disease as a diagnostic possibility in patients with primary atypical pneumonia.