The syndrome of apparent mineralocorticoid excess is a recessively inherited form of low renin hypertension. The syndrome is characterised by sodium retention and hypervolemia despite low plasma renin activity and aldosterone levels. Patients with this syndrome have mutations in the 11HSD2 gene which encodes the enzyme which normally converts cortisol in the renal tubule to its inactive form, cortisone. The unconverted cortisol is thus able to bind and activate the mineralocorticoid receptor, displacing its usual ligand, aldosterone, causing the apparent mineralocorticoid excess. We have studied a patient with severe hypertension, low renin and aldosterone, and a chronic hypokalemic alkalosis at age 4. The analysis of cortisone, cortisol and their metabolites showed the specific pattern of the apparent mineralocorticoid excess. In serum and urine, there was a dramatic decrease of cortisone and its metabolite, while cortisol and its metabolites were non affected.