Primary aldosteronism comprises sporadic and genetic forms. Three forms of familial hyperaldosteronism have been described so far. In this chapter we discuss the genetic basis, clinical phenotypes, and diagnosis and therapy of familial hyperaldosteronism type I. This condition is caused by the presence of the chimeric CYP11B1/CYP11B2 gene and displays unique characteristics such as the normalization of blood pressure and aldosterone levels after dexamethasone administration, the hyperproduction of the hybrid steroids 18-hydroxycortisol and 18-oxocortisol, and an increased rate of cerebrovascular events.