The clinical efficacy and tolerability of the vitamin D3 analogues calcitriol, calcipotriol and 1 alpha, 24 dihydroxyvitamin D3 in the treatment of psoriasis have been assessed in various clinical studies. In vitro and in vivo investigations have shown interference of these compounds with epidermal growth, keratinisation and inflammation. In this study we quantified the in vivo cell biological effects during treatment of psoriatic plaques with 1 alpha, 24 dihydroxyvitamin D3. By using a flow cytometric triple labelling procedure, we could discriminate different epidermal subpopulations, permitting precise assessment of epidermal cell cycle kinetics. Twenty patients with plaque-type psoriasis were treated in a double-blind placebo-controlled left-right comparative study with 1 alpha, 24 dihydroxyvitamin D3 ointment (4 micrograms/g applied once daily) for 8 weeks. Epidermal cell suspensions prepared from keratotome biopsies taken before and after treatment were stained with TO-PRO-3 iodide (a new DNA fluorochrome) and monoclonal antibodies against keratin 10 (as a marker for differentiation) and vimentin (as a marker for inflammation), simultaneously. The flow cytometric analyses showed a significant decrease of proliferating basal keratinocytes in verum-treated lesions, whereas such a decrease was not observed in placebo-treated lesions. The amount of keratin 10-positive keratinocytes increased and the presence of vimentin-positive cells decreased in cell suspensions derived from both verum- and placebo-treated lesions, but these effects were not significant. We conclude that multiparameter flow cytometry promises to be an adequate approach to assess the interference of antipsoriatic treatments with cutaneous inflammation, epidermal proliferation and keratinisation. Topical 1 alpha, 24 dihydroxyvitamin D3 seems to exert its in vivo antipsoriatic effect mainly through an inhibition of epidermal growth.