We report a case of a young woman with an extensive, recurrent deep vein thrombosis (DVT) diagnosed by CT scan and duplex ultrasound examination. All blood investigations for etiology of recurrent DVT were normal except for serum homocysteine level, which was mildly increased. No other thrombophilic factors could be found. The three main causes of hyperhomocysteinemia are genetic defects, nutritional deficiencies and insufficient elimination. In our case a genetic defect for one of the key enzymes of homocysteine metabolism was found to be the underlying cause. Oral anticoagulation and supplementation with pyridoxine, cyanocobalamine and folate was recommended. Whether therapy with B vitamins and folate can substantially reduce the recurrence of venous thromboembolic disease remains to be established.