Four deleterious mutations are described in the gene for HSD11B2, which encodes the type 2 isoenzyme of 11 β-hydroxysteroid dehydrogenase (11 β HSD2). In seven families with one or more members affected by apparent mineralocorticoid excess, this disorder is shown to be the result of a deficiency in 11 β HSD2. Surprisingly, the patients are all homozygous for their mutation. This results from consanguinity in two families and possibly from endogamy or a founder effect in four of the other five families. The absence of compound heterozygotes remains to be investigated.