Hypertension appears to predispose to both atheroma and thrombus formation and is a risk factor for stroke and coronary artery disease. Insulin resistance and hyperinsulinaemia are also associated with hypertension, whether treated or untreated and irrespective of obesity. In an attempt to treat the possible insulin resistance in hypertension, an antidiabetic agent, metformin, which enhances glucose uptake, was given to non-obese, non-diabetic, untreated hypertensives in a pilot study. Metformin improved insulin sensitivity, decreased plasma insulin, serum cholesterol and triglycerides, increased fibrinolytic activity and markedly decreased blood pressure. These findings support the concept that insulin resistance may be important in cases of primary hypertension, i.e. those with concomitant metabolic and possibly also fibrinolytic abnormalities. Furthermore, the results indicate that insulin resistance may precede hypertension in these cases.