During a five-year period, 26 cases of acute thoracic aortic dissection were studied by M-mode and cross-sectional echocardiography prior to cardiac catheterization and aortography. The diagnosis could be made non-invasively in 20 patients. Important information concerning the extent of the dissection, and the presence or absence of aortic regurgitation or a pericardial effusion could be obtained. Negative examinations were largely confined to cases where the distal thoracic aorta alone was involved. In a further 30 cases admitted to the coronary care unit during this period in which echocardiography was performed to rule out aortic dissection, two false-positive examinations led to aortography which demonstrated aortic dilatation and aortic regurgitation only. In the remaining 28 cases, echocardiographic examination was negative and none of these patients subsequently evidenced aortic dissection. Echocardiography using both M-mode and cross-sectional techniques is a rapid and highly effective method of making an early diagnosis of thoracic aortic dissection. Cross-sectional echocardiography is complementary to the M-mode technique, enhancing diagnostic accuracy by improving structural recognition in a condition where the normal anatomy is often severely distorted.