Several studies have shown a relation between hyperhomocysteinaemia and arterial vascular disease. We looked at the association between hyperhomocysteinaemia and venous thrombosis which could be clinically important as hyperhomocysteinaemia is easily corrected by vitamin supplementation. We studied 185 patients with a history of recurrent venous thrombosis and 220 controls from the general population. Homocysteine concentrations were measured before and 6 h after oral methionine loading. We defined hyperhomocysteinaemia as the homocysteine concentration above the fasting or the postmethionine value found for the 90th percentile of the controls. Of the 185 patients with recurrent thrombosis, 46 (25%) had fasting homocysteine concentrations above the 90th percentile or the controls (odds ratio is 3.1 [1.8-5.5]). After adjustment for age, sex, and menopausal status the odds ratio was 2.0 (1.5-2.7). Similar results were found for the post-methionine value (unadjusted odds ratio 3.1 [1.7-5.5], adjusted 2.6 [1.9-3.5]). Hyperhomocysteinaemia is a common risk factor for recurrent venous thrombosis and can lead to a two-fold or three-fold increase in risk.