We used the molecular techniques of monoclonal antibody typing, plasmid analysis, and outer membrane protein profiling to subtype 159 patients' and environmental (water distribution system) isolates of Legionella pneumophila serogroup 1 from 18 institutions. The ability of these techniques to match patients' and epidemiologically linked environmental isolates from outbreaks of Legionnaires' disease at seven institutions was also compared. Two different panels of monoclonal antibodies (I and II) identified nine subtypes (one new disease-causing subtype) and six subtypes, respectively. The Bellingham 1 subtype type was the most common among environmental isolates, and the Philadelphia 1 subtype predominated among patients' isolates from all institutions except the Veterans Administration Medical Center in Pittsburgh, Pennsylvania. The source of an isolate (patient vs. environment) and its monoclonal antibody subtype were significantly associated (P less than .01). With use of the molecular techniques tested, the subtypes of patients' isolates were identical to those of epidemiologically linked environmental isolates from the same hospital.