Rare case of hemolysis following an implantation of the biological mitral valve is presented. In 1985, biological mitral valve of Ionescu-Shiley type was implanted to a 45-year female patient for post-rheumatic mitral defect. Patient was in the III NYHA class before surgery. Her health status improved after operation. Several echocardiographic examinations have shown normal functioning of the implanted valve (normal gradient across the valve, no retrograde wave). Upper respiratory infection with subsequent progressive weakness, anemia, yellow skin, and heart failure occurred in 1991. Echocardiography has shown significant increase (twice) in the gradient across the valve (max 26, mean--14 mm Hg) and retrograde wave of the II degree as well as mitral insufficiency of the II degree. Laboratory tests have shown marked anemia with reticulosis, increase in blood bilirubin, increase in LDH activity (by about ten times), and a decrease in haptoglobins. Mitral valve dysfunction with hemolysis has been diagnosed. Patient was reoperated. Intraoperatively, incompletely healed valvular ring, and lesions to the leaves such as: fibrosis, thickening and partial deficits. Biological mitral valve was replaced with Corin-Carbotest artificial valve. Postoperative course was uncomplicated.