Trichothiodystrophy is a pilar dysplasia which is characterized by the existence of brittle hair with trichoschisis, a typical pattern of transmission of polarized light and decreased levels of sulfur containing amino acids. In this report we show various aspects of the hair dysplasia and of the hair bulbs by light and scanning electron microscopy. Normal levels of cystin in the peripheral blood were associated with decreased levels of this amino acid in the hair shafts. Incorporation of radio-labelled cystine in hair follicles seemed however normal. Our results do not support the generally accepted hypothesis of a defective transport mechanism in the hair follicle. As a similar defect in other tissues (e. g. nervous system, spermatozoids...) is also thought to be responsible for the associated symptoms (e. g. nervous impairment, sterility in males...) we think it is important to lead further research in this field in order to elucidate the metabolic pathways underlying these rare clinical syndromes.