A 36-year-old North American patient has been suffering from Olmsted syndrome since early childhood. This rare inborn error of keratinization consists in pronounced, multilating keratoderma of palms and soles and associated periorificial keratoses. The patient has now been treated with etretinate for more than 6 years, resulting in a marked reduction of hyperkeratoses. Electron microscopic investigation of skin biopsies taken before and during therapy have confirmed the significant clinical improvement. Even though this retinoic acid derivative, a powerful regulating agent with a de-differentiating effect, cannot cure the basic defect, the marked hyperproliferation of the epidermis is reduced, so that terminal differentiation is delayed and thereby normalized. At the same time, the synthesis of keratinization proteins, such as keratins and keratohyalin, which is suppressed before therapy, increases. The familial connective tissue disorder existing independently of Olmsted syndrome in this patient (perhaps a mild form of Ehlers-Danlos syndrome) is not influenced by the retinoid therapy.