Three children are reported with uncombable hair syndrome, consisting of slow-growing, straw-colored scalp hair that could not be combed flat. The hairs appeared normal on light microscopy but on scanning electron microscopy were triangular in cross section, with canal-like longitudinal depressions. Oral biotin, 0.3 mg three times a day, produced significant improvement after 4 months in one patient, with increased growth rate and with strength and combability of the hair, although the triangular shape remained. The other two patients were unique in having associated ectodermal dysplasia. Their hair slowly improved in appearance and combability over 5 years without biotin therapy.