Interrelations between hypertension (HT), ischemic heart disease (IHD) and diabetes mellitus (DM) were investigated in the diabetic subjects without IHD (DM group) or with IHD (DM + IHD group) and subjects with IHD (IHD group) which were not complicated with DM. 1. The incidence of hypertension of DM group, DM + IHD group and IHD group is 40, 54, 38% respectively. 2. The incidence of hyperlipidemia of DM group, DM + IHD group and IHD group is 55, 71, 56% respectively. 3. Serum Mg levels of DM (1.9 ± 0.37 mg/dl), DM + IHD (1.8 ± 0.23) and IHD (1.9 ± 0.33) were significantly lower than that of normal control (2.3 ± 0.32). 4. Serum Mg level of poorly controlled diabetic patients is lower than that of well controlled diabetic patients. These results suggested that Magnesium deficient state is one of the cause of insulin resistancy. (Hypertens Res 1996; 19 Suppl. I: 565-568)