Trichothiodystrophy (TTD) is an autosomal recessive disorder characterized by brittle hair with reduced sulphur content, and mental and physical retardation. Numerous additional clinical features may be present, producing a very heterogeneous syndrome. Many cases exhibit ichthyosis and photosensitivity. Cells from photosensitive TTD patients show reduced DNA repair levels similar to those found in xeroderma pigmentosum. TTD patients have a short life expectancy, and no treatment is known or envisaged. We report the prenatal diagnosis of TTD in two French families, based on DNA repair measurements in trophoblasts or amniotic cells, with later confirmation by microscopic analysis of the fetal hairs. Although the DNA repair defect was less marked in the fetal cells when compared with fibroblasts from the index case, measurement of DNA repair by unscheduled DNA synthesis provided unambiguous evidence of defective DNA repair in the fetal cells. This method is therefore a suitable prenatal diagnostic test for those TTD families in which a DNA repair defect has been identified.