Up to date, the treatment of palmar/plantar hyperkeratoses presents a therapeutic problem. The known therapeutic procedures result in short-term improvement only, if any at all. In these investigations involving 68 patients suffering from palmar/plantar hyperkeratoses of different etiology, small doses of vitamin A acid locally applied, produced a striking improvement in hypertrophic lichen planus of palms or soles: the regression was complete and in most cases permanent. The skin texture of patients with genetic keratoses and callosities became normal within a few weeks: but this condition remained free of symptoms only as long as vitamin A acid was used as a maintenance dose once or twice weekly. In hyperkeratotic eczema, pityriasis rubra pilaris, and verrucae plantaris vitamin A acid locally applied was found to be unsuitable for treatment. The possible side effects of this treatment are mentioned. Several possibilities regarding the way of action of vitamin A acid are discussed.