In four consecutive patients undergoing mitral valve replacement with pulmonary autograft, a disturbing phenomenon of graft stenosis, not observed in the previous 32 patients of our series, was noted by intraoperative echocardiography on completion of surgery. It was possible to manage this complication by re-adjusting the pericardial collar in one patient, and severing retained posterior chordae in another, but in two patients the autograft had to be replaced with a mechanical prosthesis. We believe that the cause of this complication was the use, for the first time, of softer Dacron tubing material to encase the autograft, coupled with our customary retention of the mural (posterior) subvalvular apparatus. This mishap did not recur when we returned to the use of a firmer Dacron tube.