Abnormalities of the renin-angiotensin-aldosterone system (RAAS) were observed in hypertensive patients suffering from Cushing’s syndrome. In 12 patients with different etiology of Cushing’s syndrome renin substrate concentration and urinary-free cortisol, as well as the circadian rhythms of plasma cortisol, aldosterone and plasma renin activity were measured. Plasma renin substrate concentrations were found elevated in all but 1 patient, while plasma renin activity was elevated, normal or lowered. Plasma aldosterone values were found in the lower normal range. A physiological rhythm of cortisol secretion was not observed in any patient with Cushing’s syndrome, while plasma aldosterone was secreted episodically mostly within the normal range. We conclude that changes of the RAAS may not be predominantly responsive for hypertension in Cushing’s syndrome; other factors like circulating catecholamines are probably of greater importance for the pathogenesis of blood pressure elevation in hypercortisolemic patients.