X-linked hypohidrotic ectodermal dysplasia (XLHED, OMIM 305100) is the most common form among the ectodermal dysplasias, a rare group of hereditary diseases characterized by abnormal development of eccrine sweat glands, hair, and teeth. Heterozygous carriers of XLHED often manifest minor or moderate degrees of hypotrichosis, hypodontia, and hypohidrosis. ED1, the gene involved in XLHED, encodes ectodysplasin A, a new member of the tumor necrosis factor family. The majority of mutations in XLHED are missense mutations, but one-fifth are insertion/deletions. In this report, we describe the mutation analysis of a Taiwanese pedigree with XLHED. A 35-bp deletion in exon 5 of the ED1 gene was found in the 3 affected males and in 5 female carriers. Mutation analysis families with XLHED allows for genetic counseling, prenatal diagnosis, and confirmation of carrier status.