We reviewed the chest radiographs and clinical data of 30 patients in an intensive care unit with legionellosis diagnosed by significant serological findings (n = 18), microscopic demonstration of the organism in the transtracheal aspirate (n = 5) or both (n = 7) and investigated the correlation between the identification of Legionella and signs of pulmonary infection. In 13 patients legionnaires' disease was diagnosed with a high degree of confidence. Typical radiographic findings included distal air space disease, which initially appeared to be unilateral, progressing towards bilateral infiltrates. Patchy infiltration at the onset of disease was followed by consolidation. Small pleural effusions were common, while no abscess formation was observed. Characteristically, infiltration persisted for weeks even with clinical convalescence. These radiological findings correspond well with observations in otherwise healthy patients with legionnaire's disease. In 10 patients the etiology of pulmonary infiltrates could not be identified. Seven patients did not develop radiological or clinical signs of pneumonia; therefore, the serological/microscopic detection of Legionella was not interpreted as legionnaires' disease. According to the results of our investigation the diagnosis of legionnaires' disease requires radiological findings in addition to laboratory data.