The results of valve replacement with a stent-mounted antibiotic-treated aortic allograft valve are reported in 129 patients with isolated mitral valve disease. Of these patients, 70 per cent were in N.Y.H.A. Class IV. The hospital mortality rate was 3.9 percent. The cumulative complication-free rate at 5 years was only 37 percent as 21 percent died late, a further 15 percent were alive following reoperation, 4 percent had an embolic episode, 4 percent were alive with important incompetence, and 20 percent had unimportant incompetence. Proved valve failure was due mainly to detachment of the aortic wall remnant of the valve from the pillar of the rigid metal stent (16 percent incidence at 5 years) and methods for preventing this complication are discussed. Because of these complications the use of this device in the mitral position has been discontinued.