Transesophageal and transthoracic echocardiography and color flow Doppler were performed in patients with 42 normal and 20 dysfunctioning bioprosthetic mitral and aortic valves. Transesophageal echocardiography was superior to the transthoracic approach in delineating bioprosthetic valve cusps and the presence of valve thickening due to valve degeneration. In 27 clinically normal bioprosthetic mitral valves, regurgitation was demonstrated in three patients by the transthoracic approach and in seven by transesophageal study. Both transesophageal and transthoracic color flow Doppler demonstrated mitral regurgitation in 17 clinically regurgitant valves. The severity of mitral regurgitation was accurately assessed by the transesophageal study in all 13 patients who underwent angiography, whereas the transthoracic imaging underestimated valvular regurgitation in 7 of the 13 cases (54%). Bioprosthetic aortic valves were normal on clinical examination in 15 patients and were regurgitant in three others. Both transthoracic and transesophageal color flow Doppler were of equal value in observing and quantifying aortic regurgitation. In five clinically normal and regurgitant mitral and aortic valves, transesophageal color flow Doppler revealed eccentric regurgitant jets suggestive of paravalvular leak. This feature was not evident by the transthoracic approach. In conclusion, transesophageal echocardiography and color flow Doppler are superior to transthoracic imaging in estimating bioprosthetic mitral, but not aortic regurgitation, in differentiating valvular from paravalvular regurgitation, and in demonstrating thickened valves due to cusp degeneration.