From January, 1982, through February, 1983, 20 cases of nosocomial legionnaires' disease caused by Legionella pneumopila serogroup 1 were identified in two adjoining hospital buildings, UH and RH. Although the buildings housed similar patient populations and the hot water systems in each were colonised with similar numbers of L pneumophila serogroup 1, 19 cases (1-03/1000 discharges) occurred in UH and only 1 case (0-08/1000 discharges) occurred in RH. Three subtypes of L pneumophila serogroup 1 were defined by plasmid content and monoclonal antibody binding patterns; two were unique to the environmental and patient isolates from UH and one was unique to RH. The marked differences in the attack rates suggest that the UH subtypes may be more pathogenic than the RH subtype and that the subtyping of L pneumophila might be a useful tool for studying the epidemiology of legionnaires' disease. (Copyright (c) the Lancet, September 17, 1983.)