A male German shepherd pup had symmetrical areas of hairlessness as well as missing and misshapen teeth. There was no family history of a similar phenotype. In biopsies of the hairless skin and foot pads there were no hair follicles, adnexal structures, or eccrine glands. These findings resemble those in ectodermal dysplasia in the Tabby mouse and anhidrotic/hypohidrotic ectodermal dysplasia (HED) in man, which are both X-linked recessive disorders and thought to be homologous gene defects. While similar cases of ectodermal dysplasia have been reported in the dog and some genetic studies carried out, definitive confirmation of X-linked inheritance of canine ectodermal dysplasia is lacking. Family studies and experimental matings using the propositus gave results that confirm X-linked recessive inheritance. On statistical grounds, it is concluded that ED in the propositus is due to a new mutation. A colony of dogs with this mutation is maintained for further study.