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Objective: To determine causes of treatment resistance in patients with refractory hypertension, and to estimate the prevalence of true resistant hypertension., Methods: We studied 50 consecutive patients referred with refractory hypertension after exclusion of hypokalemia and stenosis of the renal artery. Ambulatory blood pressure monitoring was performed in all patients to detect white-coat effect. The patients were hospitalized, antihypertensive drugs were withdrawn and a screening for secondary hypertension was performed. In addition, these patients, and a control group of essential hypertensives controlled with three antihypertensive drugs, underwent a OGTT with 75 g of glucose., Results: Primary normokalemic hyperaldosteronism was diagnosed in seven patients. Two patients had a pheochromocytoma and six had white-coat effect. The 35 remaining patients with true resistant hypertension shown significant differences in serum insulin and HOMA IR when compared with the control group., Conclusions: These findings show that among normokalemic treatment-resistant hypertension, the presence of hyperaldosteronism and pheochromocytoma is quite high. Moreover, treatment resistance in hypertensive patients appears to be associated with insulin resistance.