THE SYNDROME of hereditary anhidrotic ectodermal dysplasia, which has as its most prominent characteristics anhidrosis, hypodontia or anodontia and hypotrichosis, is one of a group of approximately 200 primarily cutaneous congenital abnormalities that are described in Cockayne's1 extensive study of this subject. Of this group, the following five types of abnormalities were studied and described by one of us (B. Y. U.) in a Mayo Foundation thesis, "Congenital Ectodermal and Mesodermal Dysplasias; A Clinical and Pathological Study": (1) anhidrotic ectodermal dysplasia, (2) hidrotic ectodermal dysplasia, (3) erythroderma ichthysiforme, (4) pachyonychia congenita and (5) Ehlers-Danlos syndrome. In our experience, each and all of these five types of abnormalities can well be called rare; less than 40 patients affected by them have been encountered at the Mayo Clinic in the past twenty-seven years. This paper will be confined to the study of the 4 patients affected by anhidrotic ectodermal