BACKGROUND AND OBJECTIVE: Hyperhomocysteinemia, due to a combination of genetic and environmental factors, is considered to be a risk factor for vascular disease. Individuals with the thermolabile variant of methylenetetrahydrofolate reductase (MTHFR), due to homozygous C677T MTHFR gene mutation, have significantly raised plasma levels of homocysteine and may be at increased risk of vascular disease. However, it is still controversial a direct association between C677T homozygosity and the occurrence of vascular disease is still controversial. DESIGN AND METHODS: To clarify the contribution of C677T MTHFR mutation in arterial occlusive disease (AOD) or venous thromboembolism (VTE), we performed a case-controlled study including 160 cases with AOD and 180 cases with VTE attending our referral center and compared them with 200 matched healthy controls. MTHFR gene mutation was evaluated by PCR and odds ratios (OR) and the 95% confidence intervals (CI) were used to estimate the risk for venous or arterial thrombosis. RESULTS: There was a high prevalence of homozygotes for the mutated MTHFR allele among the whole group of cases with arterial disease (OR = 2.35, p = 0.001). Considering the AOD cases with and those without associated risk factors for arterial disease separately the difference remained significant only in the latter group (p = 0.168 and P<0.001 respectively). In contrast, the prevalence of mutated homozygotes among the whole group of cases with VTE was not significantly different from that in the control group (OR = 1.67; p = 0.070). Excluding VTE cases with inherited thrombophilia or with circumstantial risk situations the value increased in both subgroups (OR = 2.26; p = 0.006 and OR = 2.03; p = 0.033 respectively). Considering only VTE cases with neither inherited thrombophilia nor circumstantial risk situations the risk increased further (OR = 2.57; p = 0.017). INTERPRETATION AND CONCLUSIONS: These data suggest that in selected patients homozygosity for the MTHFR mutation increases the risk of both arterial and venous thromboses and that differences in selection criteria for the patient group may be responsible in part for the controversial association of the MTHFR mutation and vascular disease.