The role of Legionella sp as an important pathogen, although reported in many countries, had not been well documented in Brazil. The main objective of the present study is to detect this organism or it's immunological response in patients with pulmonary infections in the city of São Paulo. For this purpose, specific laboratory methodology was introduced to cultivate the agent and demonstrate specific antibodies by serology. Patients from two University centers in São Paulo were studied: 100 from the Hospital Universitario, University of São Paulo with general pulmonary infections and 100 from Hospital das Clínicas, Faculdade de Medicina, University of São Paulo. The latter were selected to exclude pulmonary infections of other bacterial aetiology, and 30 of these were selected from the Renal Transplant Unit. Clinical specimens for cultures included sputum tracheal secretion, pleural, fluid, bronchial washing or lung tissue biopsy. Isolation of Legionella sp was attempted in BCYE medium with and without antibiotics, presumptive diagnosis by biochemical methods and identification through direct immunofluorescence staining and slide agglutination test. Direct evidence of the organism in tissue was attempted by direct immunofluorescence staining. Specific antibodies for Legionella pneumophila serogroup 1 were tested by the indirect immunofluorescence assay. Serology was also carried out in specimens from human contacts with Legionnaires' Disease. Legionella pneumophila serogroup 1 was isolated from two patients, demonstrating the presence of the pathogen in this country. Serology was able to establish present or past infection with the agent in 16 of the 100 patients from Hospital das Clínicas and in only one from Hospital Universitário. In patients considered as high risk groups from Hospital das Clínicas with renal transplantation, serology for present or past Legionellosis was positive in 33% and in 8.5% for community acquired infections. Serology in specimens from Medical personnel in contact with patients of Legionnaires' disease was positive for past infection in one single subject, confirming information already published that direct transmission from person to person is unusual. The introduction of specific methods for laboratorial evidence of Legionella sp infections at the Hospital das Clínicas-Faculdade de Medicina USP community has permitted diagnosis in able time to allow use of specific anti-microbial therapy, with good results.