Trichothiodystrophy is a congenital dysplasia of hairs characterized by: a pathognomonic image of the hair shaft under polarized light (the hair is plaited with alternately dark and light oblique bands) and trichoschisis with a clear-cut break; a deficiency of sulphurated aminoacids in the hair. We report a case where trichothiodystrophy was associated with abnormalities of the nails, teeth and eyes, growth and mental retardation, neurological syndrome and hypogonadism. From this case and a review of the cases published so far the following points emerge: 1. Congenital abnormalities of the neuroectodermal system are frequent. They include: skin lesions, such as ichthyosis, photosensitivity and atopic eczema; ungueal dysplasia; dysmorphic syndrome; growth and mental retardation; hypogonadism; neurological and ophthalmic abnormalities. 2. The condition seems to be transmitted as an autosomal recessive trait. A common element, dysplasia, enables us to include in the same nosological group as trichothiodystrophy a number of other pathological conditions, viz.: syndromes hitherto described under various names but almost identical, such as Brown's syndrome and BIDS syndrome (brittle hair, intellectual deficit, decreased fertility, small stature); IBIDS and PIBIDS syndromes which, in addition to the former, comprise ichthyosis and photosensitivity, as well as Tay's syndrome (characterized by the presence in all cases of ichthyosis associated with hair dysplasia); and perhaps some anecdotic cases with either a pathognomonic image under polarized light or a suggestive biochemical profile. Finally, since typical trichothiodystrophy has been reported in patients with Siemens' syndrome or with Marviesco-Sjögren syndrome, these two syndromes may perhaps also be classified in the same category, as borderline forms of the disease.