From July 1970 through December 1974, 109 patients underwent isolated mitral valve replacement (MVR). A Harken prosthetic disc valve (DVR) was used in 53 patients and glutaraldehyde-preserved Hancock porcine xenograft (PVR) in 56 patients. The functional class, distribution of mitral valve pathology, and incidence of atrial fibrillation were similar in both groups. There were no operative and three (5.5%) late deaths in the PVR group and two (3.8%) operative and ten (19.5%) late deaths in the DVR group. Anticoagulants were not used in the PVR group; there were 3 nonfatal emboli (10%), all occurring in patients with atrial fibrillation and large left atria. Although anticoagulants were used there were 14 emboli (five in patients who died, nine nonfatal) in the DVR group (26.4%). In our experience, there is a significant reduction in morbidity and mortality, primarily from a reduced risk of thromboemboli, if a porcine valve is used for MVR. Anticoagulants should be used in patients with atrial fibrillation and enlarged left atria regardless of the type of valve used.