Marie Unna congenital hypotrichosis (MUCH) is a rare autosomal dominant condition in which abnormalities are confined to hair shaft structure and hair density. We report a six-generation pedigree consisting of 59 members of whom 16 are affected; nine identified affected individuals are living. Affected individuals are born with adequate, normal to coarse hair. During early infancy the scalp hair becomes more coarse and wiry and stands out from the head. All affected individuals have sparse to absent eyebrows, eyelashes and body hair including secondary sexual hair. In some individuals, scalp hair is progressively lost beginning at puberty or beyond, until only a sparse fringe in the tonsorial distribution remains. The hair shafts are uniformly increased in diameter, measuring up to 0.12 mm. Individual hair shafts are deeply pigmented, variable in diameter, twisted, and bent at odd angles; some have a longitudinal groove visible on scanning electron microscopy. Cross-sectional shapes are variable and irregular, exhibiting oval, angular to reniform shapes. Multiple anagen hairs are extractable on gentle hair pull. Other ectodermal structures are unaffected except for exceptionally widely spaced upper incisor teeth seen in 50% of affected individuals. Histologically, there are dramatically reduced numbers of follicles per unit area, averaging nine total hairs per 4 mm cross-section as compared with a normal of 40. A mild to moderate inflammatory infiltrate is present, but little fibrosis and no scarring. The mechanism of progressive hair loss is unknown.