In order to determine optimum anticoagulation levels for the Medtronic Hall valve, the effect of low anticoagulation (mean International Normalized Ratio [INR] 2.5, 1979-1984) and moderate anticoagulation (mean INR 3.0, 1985-1989) was determined in 345 patients (183 low, 162 moderate) undergoing isolated mitral valve replacement (MVR) and 241 patients (91 low, 150 moderate) undergoing isolated aortic valve replacement (AVR). There were no cases of valve thrombosis. Embolic events and bleeding events were graded in severity and multiple decrement event-free survival calculated according to valve site and anticoagulation level: MVR low, MVR moderate, AVR low, and AVR moderate. In the MVR low group, 80% were free of all events and 93% free of serious events at 3 years compared with 89% and 98%, respectively, in the MVR moderate group. The AVR low group experienced a very small incidence of embolic events (one only) and no bleeding events. The AVR moderate group suffered more bleeding and more embolic events and at 3 years only 87% were event-free compared with 99% in the AVR low group. In both AVR groups, all embolic events were associated with one or more known stroke risk factors. Patients under 70, in sinus rhythm who were normotensive and were nonsmokers suffered no embolic events irrespective of their anticoagulation level. We conclude that the optimum INR for the average Medtronic Hall patient is 3.0 after MVR and 2.5 after AVR but some adjustments may be required in relation to stroke risk factor analysis.