A flow-cytometric study was performed in monogenic disorders of keratinization, to assess DNA distribution as well as the expression of keratins and involucrin. In addition, the changes in expression of these markers under influence of calcipotriol treatment were investigated. Proliferation, measured by the percentage of epidermal cells in SG2M-phase of the cell cycle, was increased in Darier's disease, lamellar ichthyosis, congenital bullous ichthyotic erythroderma of Brocq and the Comel-Netherton syndrome, whereas normal proliferation was found in autosomal dominant ichthyosis vulgaris, X-linked recessive ichthyosis, keratosis pilaris, ichthyosis bullosa of Siemens and the Sjögren-Larsson syndrome. Keratin 6 was enhanced in erythrodermic lamellar ichthyosis, congenital bullous ichthyotic erythroderma of Brocq and the Comel-Netherton syndrome, showing also reduction of keratin 10. Involucrin was only slightly reduced in erythrodermic lamellar ichthyosis, congenital bullous ichthyotic erythroderma of Brocq and the Comel-Netherton syndrome, compared to the pronounced reduction in all other skin disorders studied. Calcipotriol was found to enhance differentiation in Darier's disease, erythrodermic lamellar ichthyosis, and congenital bullous ichthyotic erythroderma of Brocq. Only Darier's disease did not show clinical improvement. In conclusion, flow cytometry provides a useful method for quantification of epidermal cell characteristics in monogenic disorders of keratinization. Further studies need to be performed to establish its usefulness as a diagnostic and prognostic tool.