Increased risk of arterial and venous thromboembolic disease is well documented in the homocystinuric patient. There is growing evidence that hyperhomocysteinemia is an independent risk factor for premature arteriosclerotic disease, including cerebral, peripheral, and coronary vascular diseases. So far no association has been established between hyperhomocysteinemia and venous thromboembolism. We studied 35 patients, young adults (age less than 56 years) with venographically and/or ultrasonographically verified deep venous thrombosis (DVT). Patients with coexisting diseases were excluded. Plasmahomocysteine levels before and after intake of methionine were measured 3 months or more after the time of diagnosis and compared with 39 control subjects. We found no significant difference in plasmahomocysteine levels between the young adults with deep venous thrombosis and control subjects. This indicates that hyperhomocysteinemia is not a frequent cause of DVT.