Objective:To study the optimal intensity of warfarin therapy for prevention of thromboembolism in elderly patients with nonvalvular atrial fibrillation and to determine the safe range of INR.Methods:Warfarin was used for anticoagulant therapy for thirty-seven patients with nonvalvular atrial fibrillation.The patients were followed up and INRs were measured regularly.Results:When the dosage of warfarin changed from 2.11 milligram to 2.41 milligram

the range of INR was between two and two point five (2.0～2.5).When the dosage changed from 2.41 milligram to 2.70 milligram

the INR was between two point five and three (2.5～3.0).No atrial fibrillation related thromboembolism and major hemorrhage occurred in all patients.It required different number of days to reach the safe INR limits by raising or lowering the dosage of warfarin in the patients.Conclusion:Warfarin is effective in preventing stroke in elderly patients with nonvalvular atrial fibrillation.The optimal intensity of warfarin ranges from 2.11 milligram to 2.41 milligram and the safe range of INR is between two and two point five (2.0～2.5).