Electron microscopic details in psoriatic skin during calcipotriol application are reported. Ten psoriatic patients (PASI score 3-14) were punch-biopsied on typical lesions before treatment and 2, 4 and 8 weeks after it. Two patients dropped out after 1 month because of lack of clinical response. Skin blocks were processed routinely for transmission electron microscopy and freeze-fracture techniques. The first modifications were the disappearance of fine intercellular granular material and the restoration of the granular layer. After 4 weeks desmosomes appeared in greater amounts than at baseline. Horny layer lipid vacuoles and remnants of nuclei as well as multilayering of the basal lamina were only focally observed. Dilatation of dermal capillaries was still seen even after 8 weeks. Freeze-fracture confirmed the data about the desmosomes and revealed a marked decrease of the gap junctions. Calcipotriol basically seems to act similarly to other antipsoriatic agents, though faster. The persistence of microvasculature dilatation could imply the need for long-term therapy.