Venous thromboembolism is associated with more than 300,000 hospitalizations and results in thousands of deaths annually. Conventional therapy consists of intravenous heparin followed by oral anticoagulants usually given for three to six months. The recommended intensity of oral anticoagulants (warfarin) has been derived from clinical trials. Such therapy is usually quite effective. However, some patients develop recurrent disease after the oral anticoagulants are stopped. A recent randomized study evaluated the optimal duration of oral anticoagulant therapy. After acute treatment with heparin, subjects were treated with oral anticoagulants for either six weeks or six months with a target INR * of 2 to 2.85. There was no difference in mortality in the two groups. Recurrence was not seen while the patients were under treatment. When anticoagulants were stopped, recurrent thrombosis was documented in 18 percent of the patients treated for six weeks and in 9.5 percent of those treated for six months. The period of greatest risk of recurrence for the six weeks patients was immediately after therapy was stopped. There was a linear increase in cumulative risk of 5 to 6 percent per year for both treatment groups during the following 18 months.