The association of hypertension with insulin resistance has been reported. Troglitazone (CS-045) is a newly developed antidiabetic agent that enhances insulin sensitivity. Its antidiabetic effects have been confirmed in diabetic animals and patients. The present study was performed to evaluate whether the amelioration of hyperinsulinemia by troglitazone lowers blood pressure in essential hypertensives. Troglitazone was administered orally to 18 outpatients with essential hypertension complicated by mild diabetes at a dose of 200 mg twice a day for 8 weeks. Blood pressure was decreased from 164 +/- 3/94 +/- 2 mm Hg to 146 +/- 3 (P < .001)/82 +/- 3 (P < .05) mm Hg at 8 weeks of the treatment period. Pulse rate did not change. Fasting plasma glucose changed from 159 +/- 10 mg/dL to 144 +/- 14 mg/dL at 8 weeks (P < .05). Plasma insulin (IRI) levels changes from 9.1 +/- 1.2 microU/mL to 6.3 +/- 0.8 microU/mL at the endpoint of treatment (P < .1). Decrease in mean blood pressure from the control period to the endpoint of the treatment correlated significantly with decrease in IRI (r = 0.59, P < .05). In summary, troglitazone treatment induces improvement in both glucose metabolism and blood pressure control in essential hypertensive patients with diabetes mellitus. These results suggest that insulin resistance or plasma insulin level plays a role in the pathogenesis of essential hypertension.