The Bioflo pericardial bioprosthesis was introduced to improve the durability of previous pericardial bioprostheses by means of a protective sheath of bovine pericardium around the stent. It has since been withdrawn from the market. Before its withdrawal, we studied 19 patients with mitral Bioflo valves (three of 25 mm, seven of 27 mm, eight of 29 mm, one of 31 mm) and 21 patients with aortic Bioflo valves (eight of 19 mm, nine of 21 mm, four of 23 mm) between two weeks and six months postoperatively, at which time all were in NYHA functional classes I or II. Standard Doppler echocardiographic measurements were taken to determine the effective orifice area of all valves, the peak and mean diastolic gradients across the mitral valves, and the peak and mean systolic gradients across the aortic valves. The smaller Bioflo valves performed poorly both in absolute terms and in comparison with other valves, presumably due to a reduction in the effective orifice area by the protective sheath. This finding should be borne in mind by the designers of future pericardial bioprostheses.