Echocardiography was performed on 26 patients with prosthetic Björk-Shiley aortic valves (BSAV). Technically satisfactory records were obtained in 22 patients. The systolic displacement of the valve disc varied from 1 to 1.8 cm. (mean, 1.4 cm.). Six patients developed valve dysfunction. Two patients had complete absence of Björk disc motion and dense echoes in the aortic root were noted on the echocardiogram. Extensive clot formation in and around the valve was seen at operation. After clot removal, these two echocardiograms showed excellent disc motion (1.5 and 1.8 cm.). In a third patient with a clotted valve, marked reduction of the velocity of opening and closing of the valve was noted. Three patients had aortic incompetence without evidence of clot formation. Normal disc motion (DM) was observed in all three. A seventh patient presented with a low output state and markedly reduced valve clicks. Echocardiography revealed normal DM with marked dilatation of the left ventricle. These features were confirmed by angiography. We conclude that: (1) satisfactory echocardiograms can be obtained in most patients with BSAV; (2) echocardiography may be a useful method of detecting Björk valve dysfunction.