Two-dimensional (2-D) echocardiographic studies were performed in 5 patients (2 adults, 3 pediatric cases) with proved aneurysm of the main pulmonary artery (PA). Two-dimensional echocardiography correctly localized the site of the aneurysm in all 5 patients and accurately categorized the type of aneurysm as saccular in 2 patients and fusiform in 3 patients. Fusiform aneurysms were characterized by their huge size, which made it impossible to image them completely in any single 2-D sector plane in 2 patients and by their tendency to cause displacement of the left atrium in 1 patient. Two-dimensional echocardiographic diagnosis of PA aneurysm was suggested prospectively without knowledge of angiographic findings in 3 of 5 patients. Two-dimensional echocardiography appears to be a useful noninvasive technique in the recognition of PA aneurysms. © 1984 Technical Publishing, a Division of Dun-Donnelley Publishing Corporation, a Company of The Dun & Bradstreet Corporation.