ObjectiveTo determine whether there is a longitudinal relationship between hypertension and hyperinsulinemia and to find the most useful parameter(s) for predicting the subsequent development of hypertension.Subjects and methodsThe oral glucose (75 g) tolerance test (OGTT) was performed in 313 patients, who were divided into three groups according to glucose tolerance based on the WHO criteria: normal, borderline and diabetes mellitus. The fasting insulin (IRI) levels, ΣIRI (the sum of the insulin levels 0, 30, 60 and 120 min after the OGTT), insulinogenic index and Homa's index, a candidate for the simple assessment of insulin sensitivity, of the normotensive and hypertensive subjects in each subgroup were compared. In addition, 145 normotensive subjects were followed up for over 3 years and observed for the development of hypertension.ResultsHypertensive diabetic subjects had not only higher fasting IRI levels and ΣIRI values, but they also had higher Homa's indices than normotensive diabetics. Normotensive subjects with normal glucose tolerance (n = 20) did not develop hypertension. However, 16 out of 94 patients with borderline glucose tolerance and five out of 31 diabetics became hypertensive. The incidence of hypertension in the group with fasting IRI ≥ 15, ΣIRI ≥ 150 or Homa's index ≥ 4 was between 5 and 9 times higher than that in the group with fasting IRI < 10, ΣIRI < 100 or Homa's index < 2. This difference was still significant when multivariate analysis, including various factors such as age, body mass index (BMI) and sex, was performed.ConclusionsThese results suggest that higher plasma IRI levels and/or insulin resistance are closely related to the pathogenesis of hypertension in patients with diabetes mellitus. Homa's index, fasting and ΣIRI may be useful predictors of the subsequent development of hypertension.