One hundred and six consecutive patients who had mitral valve replacement with either a St. Jude or porcine heterograft prosthesis were prospectively studied. The 2 groups are similar with respect to 67 clinical and operative factors and allow comparison of valve performance as an independent variable. Total follow-up is 3,312 patient-months (mean 36 months, range 2-57 months, 94% complete). There are no statistical differences in symptomatic improvement or mortality by life table analysis. Valve-related complications expressed as percent per patient-year are: reoperation: 1.8 St. Jude and 3.8 porcine; endocarditis: 1.2 and 1.9; regurgitant murmur: 2.3 and 1.9; hemolysis: 1.8 and 0.0; late thromboembolism: 1.8 and 1.0; hemorrhage: 2.9 and 2.9; and valve failure: 0.0 and 1.0. There were no significant differences found. Actuarial survival at 3 years was 78% in St. Jude and 81% in porcine patients. Forty-six percent of patients with St. Jude valves and 55% of patients with porcine valves were alive and free of all complications at latest follow-up. The clinical performance of St. Jude and porcine mitral valves are similar over this period of intermediate follow-up.