An 18-month-old and an 8-year-old girl had trichothiodystrophy (TTD). Microscopic observation of the hair under polarized light showed typical alternation of bright and dark bands; amino acid analysis of the hair demonstrated a marked reduction of cystine levels. Both patients had skin lesions consisting in the older child of diffuse follicular keratosis since birth, and in the younger of an ichthyosiform dermatitis on the lower legs that appeared at age 4 months. Ultrastructural studies of the skin showed striking similarities in both cases: perinuclear vacuoles with a unit membrane in the keratinocytes, and dispersed, irregularly arranged bundles of tonofilaments particularly at the desmosome junction. The origin of the vacuoles is unknown; the abnormalities of the tonofilaments could be explained on the basis of a generalized abnormality in sulfur-containing proteins, reflecting a disturbance in the synthesis of keratins. These electron microscopy findings could be considered as a peculiar feature of ichthyotic skin in patients with TTD.