Essential Hypertension (EH) is correlated with a metabolic disturbance characterized by insulin resistance (IR). In this study, there were observed in 47 subjects with EH and 30 subjects with normal blood pressure. Serum levels of insulin-like growth factor-1 (IGF-1), serum levels of growth hormone (GH), the activity of erythrocyte insulin receptors (EIR), and ATP levels in erythrocytes, the insulin sensitivity index (ISI) was used to study the correlative factors of essential hypertension. 1. Among patients with EH, ISI, GH, and low-affinity insulin binding sites of EIRs (RT₂) were found to be in significantly lower amounts, IGF-1 levels and the KD₂ of the erythrocyte insulin receptors were noted to be significantly higher. Compared with the control group, there was a marked difference between EH group and the control group. However, no statistical difference was observed between the hypertensive group and the group with normal blood pressure as regards erythrocyte ATP levels, high-affinity insulin binding sites of EIRs (RT₁), and the KD₁ of EIRs. 2. In the hypertensive group, the ISI was negatively correlated with mean arterial blood pressure (MBP), a family history of hypertension, the body mass index (BMI), the waist-hip ratio (WHR) and IGF-1 levels (r=-0.614^Δ, -0.354^**, -0.386^**, -0.472^**, -0.298^*, ^Δp<0.001, ^**p<0.01, ^*p<0.05), were positively correlated with RT₂ and GH levels (r=0.301^**, 0.275^*, ^**p<0.01, ^*p<0.05). There were no statistically significant differences between ISI and age, sex, smoking history, drinking, RT₁, KD₁, and ATP levels in erythrocytes. 3. The ISI was used as the dependent variable in multiple linear stepwise regression analysis. MBP (X₁), a family history of EH (X₂), WHR (X₃), GH (X₄), IGF-1 (X₅), RT₂ (X₆), and the body mass index (X₇) was used as independent variables. X₁, X₂, X₃, X₅, X₆, and X₇ were used in the equations. The results indicate that patients with EH also tend to have IR. We suggest that MBP, a family history of hypertension, BMI, WHR, IGF-1, and RT₂ might be independent factors affecting IR in cases of essential hypertension. (Hypertens Res 2000; 23: 265-270)