GLUCOCORTICOID-REMEDIABLE Received 26 September; accepted 29 October 1991.aldosteronism (GRA), an autosomal dominant disorder, is characterized by hypertension with variable hyper aldosteronism^1,2 and by high levels of the abnormal adrenal steroids 18-oxocortisol and 18-hydroxy-cortisol^3–5, which are all under control of adrenocorticotropic hormone and suppressible by glucocorticoids⁶. These abnormalities could result from ectopic expression of aldosterone synthase, which is normally expressed only in adrenal glomerulosa, in the adrenal fasciculata. Genes encoding aldosterone synthase^7–9 and steroid 11β-hydroxylase⁷(expressed in both adrenal fasciculata and glomerulosa), which are 95% identical⁷ and lie on chromosome 8q (refs 7, 10), are therefore candidate genes for GRA. Here we demonstrate complete linkage of GRA in a large kindred to a gene duplication arising from unequal crossing over, fusing the 5' regulatory region of 11β-hydroxylase to the coding sequences of aldosterone synthase (maximum lod score 5.23 for complete link-age, odds ratio of 170,000:1). This mutation can account for all the physiological abnormalities of GRA. Our result represents the demonstration of a mutation causing hypertension in otherwise phenotypically normal animals or humans.