The pulmonary artery is the second most common site of arterial involvement in Behcet's disease. A 32-year-old man presented with bilateral ankle edema, abdominal discomfort, and hemoptysis. He had a history of recurrent oral and genital aphthous ulcerations for 1 year. The diagnosis of Behcet's disease was made on the basis of the criteria published by the International Study Group for Behcet's Disease. His chest X-ray revealed left hilar enlargement. A helical computed tomography (CT) scan showed a pulmonary aneurysm with intramural thrombosis in the left pulmonary artery and enlarged hepatic veins. Treatment with colchicine and cyclophosphamide was given for 24 months, and helical thoracic CT was performed again. Helical CT showed that the pulmonary aneurysm was reduced by treatment. Helical CT could be used in Behcet's disease for the diagnosis and follow-up of pulmonary involvement.