From june 1974 to december 1975, 204 Hancock prosthetic valves were engrafted. The in-hospital mortality was 8.7%. The eight year survival rate was 78.9%. Of the 171 patients who had surgery, 121 reverted to functional class I, 123 had a mild or absent cardiomegaly. After 8 years of follow up 92% of the cases had not thromboembolic episodes. Ninety five percent of the patients with mitral valve replacement and 95% with aortic valve replacement were free of bacterial endocarditis. During the follow-up period; there were 49 valvular malfunctions, 16 of them mild that required no treatment. In 32 cases the alteration was severe and required surgical intervention. Thirty six prosthetic valves had to be removed; 22 of them had degenerative changes, 10 had bacterial endocarditis and in 4 the suture line broke down. After 8 years 77% of the valves were functioning well. The in-hospital mortality after the second valve replacement was 9.4%.