The authors report the first two cases of legionnaires' disease from Catalonia. Both patients were chronic bronchitic males, and the cases were sporadic. The onset of the disease was characterized by a febrile illness with muscle and joint pains, respiratory symptoms (cough and mucous sputum production), and mental changes. There were no digestive complaints. Pulmonary consolidation occurred in both patients in the left upper lobe. Blood chemistries disclosed the existence of an absolute lymphopenia, altered liver function tests, and elevated CPK levels. Bacterial cultures of blood and sputum, respiratory virus screening (influenza A and B, parainfluenza 1, 2 and 3, and adenoviruses), and tests for Mycoplasma pneumoniae, Coxiella burnetti and Chlamydia psittaci were all negative. Antibody titers against Legionella pneumophila by indirect immunofluorescence were 1/1024 (positive) for serotype 1 and 1/1024 (positive) for serotype II in one patient, and 1/1024 (positive) for serotype I and 1/128 (negative) for serotype II in the other patient. The authors review the epidemiological, clinical, biochemical and diagnostic aspects of legionnaires' disease, which knowledge will undoubtedly allow to detect an increasing number of cases.