Valve dysfunction can be caused by thickening or contraction of a fibrous sheath covering a cusp of a porcine bioprosthesis, but this is uncommon. This complication appears to more frequent in other bioprostheses, such as fascia late valves and homografts, in which fibrous sheaths seems to grow more rapidly. rapidly. Thus the slow and limited growth of fibrous sheath in porcine bioprostheses is advantageous in this respect. Recently, we experienced a case of valve dysfunction caused by fibrous tissue overgrowth on Hancock mitral xenograft in 45 year old female. 3.5 years ago, the patient was received valve replacement due to mitral stenoinsufficiency. But since 2.5 years elapsed after operation, she has complained of generalized edema and dyspnea, and their symptoms were aggravated progressively. So reoperation was performed under the diagnosis as valve dysfunction of mitral xenograft and newly developed tricuspid insufficiency. Her postoperative courses were good.[KTCS 1983;4:506-510]