Aims/hypothesis. Although hyperhomocysteinaemia and methylenetetrahydrofolate reductase gene polymorphism are accepted risk factors for cardiovascular disease, their association with micro angiopathy or blood pressure in diabetic patients is still being debated. This study explores the relation between plasma homocysteine concentrations, methylenetetrahydrofolate reductase gene polymorphism, hypertension, diabetic microvascular and macrovascular complications associated with kidney function. Methods. Vascular complications, hypertension, methylenetetrahydrofolate reductase genotype (RFLP with Hinf I digestion), and total plasma homocysteine (HPLC) were investigated in 389 well-characterized Type I (insulin-dependent) diabetic patients with normal (GFR±75 ml·min-1·(1.73 m2)-1; n=273), or impaired renal function (GFR<75 ml·min-1·(1.73 m2)-1; n=116). Results. Patients with microvascular and macrovascular complications showed higher total plasma homocysteine concentrations than those without complications. However, after the data for GFR (main determinant for plasma homocysteine) was adjusted we observed that plasma homocysteine concentrations greater than 8.6 μmol/l in patients with normal GFR are not related to vascular complications, but to hypertension (8.6-11.3 μmol/l: OR 1.9; >11.3 μmol/l: OR 3.7). The risk for coronary heart disease (CHD) was also enhanced by a plasma homocysteine concentration greater than 11.3 μmol/l (OR 5.9). Although the T allele was an independent determinant of plasma homocysteine, the methylenetetrahydrofolate reductase gene polymorphism was neither associated with diabetic vascular complications nor with hypertension. Conclusion/interpretation. Increased plasma homocysteine concentrations but not the T allele per se, enhance the risk of hypertension and of CHD in Danish Type I diabetic patients with normal renal function.