Elevated plasma total homocysteine concentration is a risk factor for venous thrombosis. The association is well established in patients with homocystinuria irrespective of the genetic etiology and metabolic background. Homocystinuria is a human model of chronic exposure to very high concentrations of plasma homocysteine and reflects an abnormal amino acid metabolism. Elevated homocysteine levels in patients with venous thrombosis have attracted considerable interest because homocysteine is a potentially reversible thrombophilic marker for venous thrombosis. In contrast to homocystinuria, hyperhomocysteinemia is mild and reflects environmental and constitutional factors such as age, intake of B-vitamins, derangements of metabolism, and renal impairment. This review examines the evidence for the relationship of homocysteine with risk of venous thrombosis in homocystinuria and in the general population.