Trichothiodystrophy (TTD) is a rare autosomal recessively inherited disorder which is characterized by sparse and brittle hair with low cystine content. It is often associated with physical and mental retardation. We report 2 cases of TTD in 2 sibs who were born to related parents. The children showed clinical features typical of TTD and in addition other symptoms such as epilepsy, ataxia, spasticity, strabismus, atopic dermatitis, dysarthria and hyperextensible fingerjoints. The sulfur content of hair was reduced to about 50% of normal values and scanning electron microscopy of hair showed trichorrhexis nodosa, trichoschisis, missing cuticle scales with weathering of hair shafts. Under polarizing microscopy an alternating dark and bright banding was found. The present cases show that the correct diagnosis of TTD in practice can be impeded for many years because of the heterogeneous clinical appearance and that the determination of the sulfur content in hair is a simple but indispensable method.