In the above review, the authors intended to stress the epidemiologic importance of legionellosis which has become a frequent cause of respiratory pathology in the short time since its first appearance (1976), reaching a for from negligible proportion among respiratory disorders (2-8% of acute bronchopulmonary diseases) and a considerable death rate (10-40%). Clinical and diagnostic as well as therapeutic aspects and especially prophylaxis all discussed, the latter concerning above all the environment and adequate maintenance of heating and air-conditioning equipment in community centers, and particularly in hospitals and hotels.