Pachyonychia congenita (PC) is a very rare hereditary disorder of keratinization. Immunohistological findings has so far been lacking. Reported in this paper is a case of Jadassohn-Lewandowsky type PC in a woman aged 18 years on which immunohistological investigations could be performed. Several monoclonal antibodies to filaggrin and keratin were used to stain tissue sections of lesional plantar skin, with a view to studying impairment of epidermal differentiation. While staining patterns comparable to those of normal skin were exhibited by anti-filaggrin and some antikeratins (RPN 1161, A51-B/H4), substantially altered immunostaining was recordable from other anti-keratins. Only superficial vital keratinocytes were stained by RKSE 60 against keratin 10 and K 8.12 against keratins 13 and 16. The authors, in other words, obtained information on expression of keratin 10, normally occurring in all suprabasal keratinocytes, as well as of the basal proliferation keratin 16 in the uppermost vital cell positions of PC lesion. The above results are likely to suggest impairment of keratin expression in cases of PC.