We have used detailed ultrastructural and electron histochemical techniques on both the hair shaft and anagen hair follicle to elucidate further the structural abnormalities in trichothiodystrophic hair. We have shown that protein deposition in the follicle is reduced and lacks orientation, and that the anagen follicles show an overall distortion. Both the hair cuticle and cortex high sulphur protein components are affected. The results of this study help to give a visual interpretation to detailed biochemical studies conducted by other workers thereby allowing specific localization of the site of the intrinsic keratin abnormality.