Behçet's disease, initially described as local vasculitis involving the oral, genital mucosa and eyes, can affect many organ systems with a varying clinical course. Pulmonary lesions associated with Behçet's disease are rare and reported to have a prevalence of 1.0-7.7%. In this prospective study, 29 patients aged 16-60 years (mean +/- s: 34 +/- 10.5) were investigated for pulmonary involvement with direct radiography, computerized tomography (CT) and intravenous digital subtraction angiography. Lung scintigraphy was also performed to determine its diagnostic value. In addition, patients had a fibreoptic bronchoscopic examination, spirometric test, carbon monoxide diffusion test and methacholine test. Of the 29 patients with Behçet's disease, 17 had pulmonary involvement. The diagnostic criteria for pulmonary vasculitis and tracheobronchial tree involvement were: (1) hilar vascular prominence or mass, new alveolar and reticulonodular infiltrates on chest X-ray; (2) aphthous lesions, bleeding ulcerations, inflamed mucosal appearance on bronchoscopy; (3) vascular thrombosis or aneurysms on CT; (4) occlusion or aneurysms of the main pulmonary arteries or its branches. The value of lung scintigraphy and its correlation with other imaging modalities were investigated to determine if a lung perfusion scan alone can be used to detect pulmonary involvement in Behçet's disease. Twenty-six patients were investigated with lung scintigraphy. Thirteen of 14 patients with lung involvement were identified. Perfusion lung scintigraphy provided true diagnostic information in 96% of our patients. In eight patients without pulmonary involvement, the lung scans were normal. Although DSA specifically demonstrates pulmonary aneurysms, it involves administration of contrast media and cannot show abnormalities distal to the third pulmonary arterial branches. Although vasculitis caused by collagen disorders is physiologically indistinguishable from pulmonary emboli, most perfusion defects resolve within 3 months, which can be used as a differential diagnostic criterion in the appropriate clinical setting. Therefore, pulmonary perfusion imaging can be used to screen patients suspected of having lung involvement, especially in the absence of thrombophlebitis. In patients with thrombophlebitis, a repeat examination at 3 months is recommended to differentiate pulmonary involvement from pulmonary embolism.