The clinical and plain radiographic findings of thoracic aortic dissection may not be specific; thoracic aortography is the definitive diagnostic procedure. Ulcerlike projections of contrast material are not commonly found on thoracic aortography, but may be a precursor angiographic sign of dissection. Two patients with ulcerlike outpouchings are presented. In one, the evolution of aortic dissection was documented by three serial aortograms. It began with a small "ulcer" in the ascending aorta that became larger and finally progressed into the typical angiographic appearance. In the other, thoracic aortography demonstrated an ulcerlike projection in the ascending aorta. In both cases, the diagnosis of dissecting hematoma was confirmed by surgery and pathologic examination of the resected specimens. Thoracic aortography in several projections is necessary for demonstration of these ulcerlike outpouchings because they may be evident in only one projection. Conceivably, their recognition may permit earlier diagnosis of aortic dissection and the institution of appropriate treatment.