Twenty patients with psoriasis were treated with the vitamin D3 analogue MC903 and betamethasone ointment in a double-blind trial with a left-right comparison. In addition to the clinical severity scores, Ks8.12 binding which detects keratin 16 expression and the DNA synthesis were quantified using flow cytometry. Both markers decreased significantly with treatment, but remained above the normal range even in those who had total clearance of the lesions. Treatment with MC903 with regard to Ks8.12 binding was significantly better than with betamethasone.