Insulin resistance is associated to hypertension, obesity and diabetes and may be an independent cardiovascular risk factor. The exact assessment of insulin resistance requires complex metabolic studies. However, there is a good correlation between this parameter and fasting serum insulin levels. The aim of this work was to study fasting serum insulin levels by radio immuno analysis in 43 hypertensive patients aged 56 +/- 5.5 years old (27 male, 17 obese and 8 diabetics) and 20 normotensive controls aged 50 +/- 4.8 years old (13 male). Insulin levels were 3.8 UI/L in controls, 12.1 UI/L in normal weight, 15.5 UI/L in obese and 18.3 UI/L in diabetic hypertensives (ANOVA p < 0.001). These levels were above two standard deviations of control values in 50% of normal weight, 66% of obese and 62% of diabetic hypertensives. It is concluded that normal weight, obese and diabetic hypertensive subjects have high fasting insulin levels.