Background: There are no reports about the long-term follow-up data of patients with aortic plaques among the Japanese population. The purpose of this study was to clarify the frequency of patients with severe aortic plaques and the impact of aortic plaque morphology on the prognosis. Methods and Results: We retrospectively investigated 1,570 consecutive patients who underwent transesophageal echocardiography. Survival rate and subsequent embolic event rate were compared between patients with severe aortic plaque (>5 mm in thickness) and control patients. The relationship between aortic plaque morphology and prognosis was also estimated according to the presence of ulceration, calcification, hypoechoic plaques, and mobile plaques. The mean follow-up period was 8.7 years. Among 1,570 patients, severe aortic plaque was detected in 92 patients (5.9%). These 92 patients showed a significantly low survival rate and high subsequent embolic event rate compared to control patients (5-year survival rate: 69% vs 94%; 5-year embolic event free rate: 52% vs 95%). Among patients with severe aortic plaque, only ulceration was associated with a low survival rate (hazards ratio: 2.4, 95% confidence interval (CI): 1.1-5.2) and only mobile plaque was associated with a high embolic event rate (hazards ratio: 2.2; 95%CI: 1.1-5.1). Conclusions: Aortic plaque >5 mm in thickness was a predictor of poor prognosis. In the presence of aortic plaque >5 mm, ulceration was a predictor of a low survival rate and mobile plaque was a predictor of a high embolic event rate. (Circ J 2010; 74: 2152-2157)