: Cushing's syndrome was diagnosed in a 14 year old girl and, 5 years later, in her nephew (sister's son) also when he was 14 years of age. Administration of tetracosactid (Synacthen®) did not stimulate adrenocortical function in either patient, nor did dexamethasone suppress the elevated urinary steroids in the girl, and only partially suppressed them in the boy. Both patients were adrenalectomized and made an uneventful recovery. The surgical specimens were identical both on gross inspection and histologically, showing the typical morphological features of pigmented multinodular adrenocortical dysplasia. We believe that this lesion represents an inherited adrenocortical maldevelopment (rather than true neoplasia) leading to an autonomous adrenocortical hyperfunction which typically manifests itself clinically during adrenarche (i.e. functional adrenocortical maturation) at the beginning of puberty. Familial Cushing's syndrome, caused by pigmented multinodular adrenocortical dysplasia, represents a disease entity.