Microalbuminuria is associated with excess cardiovascular mortality in both diabetic and nondiabetic subjects. Patients with NIDDM and microalbuminuria are more insulin resistant than those without microalbuminuria. However, the relationship between insulin resistance and microalbuminuria in patients with NIDDM could be due to hyperglycemia, which can cause both insulin resistance and an increase in albumin excretion rate. Little is known about microalbuminuria and insulin resistance in nondiabetic subjects. Therefore, we examined, cross-sectionally, the relationship of insulin sensitivity (S(I) x 10(-4) min x microU(-1) x ml(-1)), estimated by a frequently sampled intravenous glucose tolerance test and the minimal model and fasting plasma insulin concentration, to microalbuminuria (albumin-to-creatinine ratio > or = 2 mg/mmol) in 982 nondiabetic subjects aged 40-69 years. Altogether, 15% of the subjects had microalbuminuria, and 32% had hypertension. Subjects with microalbuminuria had a lower degree of insulin sensitivity (means +/- SE, 1.70 +/- 0.11 vs. 2.25 +/- 0.07, P = 0.003) and higher fasting insulin concentrations (17.4 +/- 1.1 vs. 15.7 +/- 0.5 mU/l, P = 0.059) compared with subjects without microalbuminuria. In logistic regression analysis, an increasing degree of insulin sensitivity was related to a decreasing prevalence of microalbuminuria (odds ratio = 0.86, 95% CI: 0.79-0.94, P < 0.001). Although this relationship attenuated after adjustment for age, sex, ethnicity, hypertension, fasting glucose, and BMI, it still remained significant. The association between insulin sensitivity and microalbuminuria was shown not to be different between normotensive and hypertensive subjects. Our results suggest a relationship between insulin resistance and microalbuminuria in nondiabetic subjects that is partially dependent on blood pressure, glucose levels, and obesity.