The purpose of this study is to compare clinically and hemodynamically the Wessex and Hancock II porcine bioprostheses. We compared functional class and data from echo-Doppler in 34 Wessex bioprostheses (group A) with those in 42 Hancock II bioprostheses (group B). We subdivided group A into A1 and A2. A1 was made up of 23 Wessex manufactured since 1986. A2 constituted 11 Wessex made before 1986 which belonged to a series with some variations in the manufacturing process, and in which some early dysfunctions have been described. We compared data from these sub-groups between each other as well as with those of group B. The groups were homogeneous in age, sex, patients body surface and the time elapsed since the prosthetic implant. The mean mitral gradient, the mitral area, the peak aortic gradient and the regurgitation incidence were similar in groups A and B. In A2 the mean mitral gradient was significantly superior to that of group B (7.1 +/- 1.1 mmHg vs 5.4 +/- 1.4 mmHg; p less than 0.01), and the mitral area showed a tendency to be inferior, although with no statistical significance. The functional class of the patients was similar in all the groups. We conclude that the Wessex bioprosthesis presents hemodynamic data and functional class similar to those of the Hancock II, with the exception of a sub-group of Wessex manufactured before 1986 which presents mean mitral gradients superior to the others and which would warrant further studies.