Many clinical trials have demonstrated the effectiveness of oral anticoagulation for the primary and secondary prevention of venous thromboembolism, myocardial infarction, systemic embolism and stroke. However, the relationship between potency and/or changes in anticoagulation therapy and frequency of complications is not clear. This study retrospectively investigated 157 patients (valvular heart disease 37, artificial valve replacement 19, atrial fibrillation 11, ischemic heart disease 55, post coronary artery bypass grafting 25, others 10: mean age 55 +/- 12 years) who received oral anticoagulation therapy for 4.9 +/- 3.2 years to investigate the development of complications. The thrombotest and prothrombin time were measured at follow-up examinations every month (mean interval 31 days). Target range of anticoagulation was 2.5 to 3.5 (international normalized ratio: INR). During the 770 patient-years of follow-up, seven major bleedings (cerebral, renal, gastrointestinal, etc.), 111 minor bleedings (subcutaneal, nasal, gum, etc. bleeding), 16 major thromboembolisms (cerebral, renal, etc. infarction) and 4 minor thromboembolisms (transient ischemic attack) were observed. INR at the onset of the complications was 2.93 +/- 0.41 in patients with bleeding and 1.81 +/- 0.46 in those with thromboembolism. INR was greater than 2.75 in all patients with major hemorrhage and less than 2.75 in those with major thromboembolism. Seventy-five percent of bleeding complications developed at the increasing phase of INR and 70% of thromboembolism at the decreasing phase. Cumulative rates free from bleeding complications with a mean INR of < or = 2.0, 2.0-2.5 and > 2.5 were 76.8, 62.8 and 45.5%, respectively, at 5 years.(ABSTRACT TRUNCATED AT 250 WORDS)