Left ventricular flow patterns were studied at rest by colour flow Doppler echocardiography in 33 patients with mitral tilting-disc prostheses (group 1), in 38 patients with mitral tissue prostheses (group 2) and in 18 healthy volunteers (controls). A 'crossed' flow pattern was seen in 14 patients with mechanical (42%) and in 15 patients with tissue prostheses (39%). The remaining patients and all controls had either 'parallel' or 'intermediate' flow patterns which were classified as 'normal'. There was a significant correlation between the type of flow pattern and the position of the mitral prosthesis in both study groups. The presence of a crossed flow pattern, however, was not related to functional status (assessed clinically by NYHA class) or to abnormalities on the electrocardiogram. Left atrial size was greater in patients with crossed flow than in those without, but all other echocardiographic parameters were similar. Eight patients with crossed and eight with normal flow patterns underwent treadmill exercise testing; there was no difference between the two groups with regard to exercise performance as determined by exercise duration, maximum oxygen consumption and the ventilatory response to exercise. The results of this study indicate that the pattern of blood flow within the left ventricle may be fundamentally altered by the orientation of both mechanical and tissue prostheses. The presence of a crossed flow pattern is not, however, accompanied by significant deleterious haemodynamic or functional consequences.