An 11-year old girl was seen in 1981 with hypokalemia, low renin, low aldosterone, and severe hypertension. A medical adrenalectomy with dexamethasone and aminoglutethimide, and the blockade of mineralocorticoid receptors with spironolactone improved her condition, but the blockade of glucocorticoid receptors with RU-486 worsened it. An aldosterone infusion induced no changes. A sister was born in 1982 with similar findings. Both patients had an impaired ability to convert cortisol to cortisone after an oral load of 200 mg cortisol. In urine, an elevated ratio for metabolites of cortisol to metabolites of cortisone was found. These data suggested a defect in the activity of renal 11ß-hydroxysteroid dehydrogenase. Both parents were asymptomatic, phenotypically normal and non-consanguineous. Their urinary metabolites of cortisol and cortisone were normal before and after stimulation with ACTH. However, the mother reached a peak plasma cortisone concentration 3 SD below the mean reached by normal subjects after an oral 200-mg cortisol load, a fact that suggests that this test could be used to detect heterozygotes. The genetic studies revealed a homozygous mutation on exon 3 of the HSD11K gene, which by substituting TGC for CGC changes Arg 213 for Cys and induces a loss of 84% of the enzymatic activity in transfected cells. Both unrelated parents had the same heterozygous mutation. Both patients have been treated with dexamethasone but have also required spironolactone. The older sister has also required high doses of nifedipine to lower her blood pressure. After 19 years of follow-up, the older sister has become normotensive and normokalemic under therapy, and reached a final height of 140 cm at age 17. The younger sister has increased her mean blood pressure at a rate of 1 mm Hg per year, in spite of treatment. Her final height is 143.5 cm. (Rev Méd Chile 2000; 128: 17-26)