Isolation of Legionella pneumophila sero group 1 with serological evidence of present infection is reported from a 40 year-old male with serious respiratory infection which developed into acute respiratory failure. It was characterized by severe hypoxemia resistant to high inspired oxygen concentrations and radiographycally by diffuse infiltrates in both lungs suggesting the clinical aspect of ARDS. Following the introduction of clindamycin, amikacin, ceftriaxone, volume-cycled ventilator and positive end expiratory pressure (PEEP) of 14 cm H(2)0, stabilization of clinical conditions and gradual recovery were achieved. Suspecting of legionellosis, blood and tracheal secretions specimens were collected for specific laboratory research. From tracheal se cretion cultivated in BCYE medium, gram-negative bacilli were isolated and identified as Le gionella pneumophila serogroup 1 through cul tural and biochemical characteristics and direct immunofluorescence and slide agglutination tests. Serology (IFA) with blood samples collecting during the 1st, 3rd, 4th and 6th weeks of illness demonstrated antibody titers to the isolated microorganism of 128, 1024, 4096 and 8192, respectively. Definitive results were obtained during the patient's recovery. The authors emphasize: a) the demonstration of the presence of Legionella sp. as a patho genie agent in Brazil; b) the importance of supportive care in the clinical outcome; c) the need of remembering this pathogen while making differential diagnosis of pneumonias and of continuing to pursue this etiology with specific laboratory methodology.