Harlequin ichthyosis (HI) is an extremely severe and usually fatal congenital keratinization disorder whose responsible genes have not yet been identified. For prenatal diagnosis, the fetal skin biopsy is the only available method and has been usually performed at 21 to 22 weeks' estimated gestational age (wEGA). Hair canal keratinization is thought to occur around 15wEGA prior to the inter‐follicular keratinization, and characteristic abnormalities of HI are known to be expressed more strongly in the hair canal. Thus, we expected the fetal skin specimen at 19wEGA to have sufficient information for prenatal diagnosis. Fetal skin biopsy was undertaken from a fetus at risk at 19wEGA. Electron microscopy demonstrated abnormal vacuoles in keratinized cells and malformation of lamellar granules in the hair canal. Clumps of aberrantly keratinized cells containing lipid droplets were seen in the amniotic fluid. The fetus was diagnosed as affected. The abortus at 21wEGA demonstrated HI phenotype clinically. The present results indicate that the prenatal diagnosis of HI is possible at 19wEGA, an earlier stage of gestation than previously reported, by the ultrastructural observation of the hair canal and the amniotic fluid cells, but not the interfollicular epidermis, of fetuses at risk. Copyright © 1999 John Wiley & Sons, Ltd.