Classical homocystinuria is associated with arterial vascular diseases and 
venous thrombosis. In the last decade, several studies have been published 
indicating that even mild hyperhomocysteinemia is a risk factor for venous 
thrombosis. The 677C-->T mutation in the methylenetetrahydrofolate reductase 
(MTHFR) gene is an important cause of mild hyperhomocysteinemia, but this 
polymorphism does not seem to be a risk factor for venous thrombosis. Studies on 
the interaction between hyperhomocysteinemia and other thrombotic risk factors 
are conflicting. Little is known about the pathophysiology of venous thrombosis 
in hyperhomocysteinemia. Several mechanisms proposed for vascular disease may be 
applied to venous thrombosis as well. However, up to now there is no satisfying 
model which might explain a thrombophilic state at plasma homocysteine 
concentrations in the range of mild hyperhomocysteinemia. The results of a first 
clinical intervention study are expected in 2002. As the results are pending, 
clinicians could perform homocysteine measurements in patients with venous 
thrombosis if screening for thrombophilia is indicated. Vitamin supplementation 
could be prescribed if homocysteine levels are elevated. However, the patient 
should be informed about the uncertainty of the benefits of vitamin 
supplementation.