Only a few longitudinal studies in Asians have shown a close association between hyperinsulinemia and the incidence of hypertension. In the present study, we used the insulin resistance index, HOMA-IR [fasting plasma glucose (FPG) (mmol/l) × insulin (μU/ml) / 22.5], and tried to elucidate longitudinally the relationship among insulin resistance, blood pressure, body mass index (BMI), FPG, and lipid profile in middle-aged Japanese men. Three hundred and ten male non-hypertensive subjects aged 30 to 58 years in 1993 (44.5±7.8 (SD) years; BMI, 22.6±2.5 kg/m²) were divided into three groups according to HOMA-IR (A: lowest; B: middle; and C: highest). Their BMI, systolic blood pressure (SBP), diastolic blood pressure (DBP), heart rate, FPG and lipid profile were followed up for 7 years. The percentage of high normal blood pressure in 1993 and 2000 did not differ among the three groups. However, in 2000 hypertensive subjects made up 11.7% of group A (12/103), 15.4% of group B (16/104), and 29.1% of group C (30/103) (χ² =11.50, p =0.003). SBP, DBP, and many other metabolic parameters in 2000 were significantly higher in group C than in the other groups. Furthermore, both SBP and DBP in 2000 were positively correlated with HOMA-IR in 1993 (r =0.17 and p <0.01 for both). The correlation between HOMA-IR in 1993 and DBP in 2000 was significant, even after adjustment for age and BMI in 1993. These findings suggest an important role of insulin resistance in predicting the future incidence of hypertension in middle-aged Japanese men. (Hypertens Res 2003; 26: 795-800)