The efficacy of MC903, a vitamin D3 analogue, in reducing hyperproliferation as determined by levels of ornithine decarboxylase (ODC) was investigated in a double-blind study of 15 patients with chronic plaque psoriasis. The lesions of psoriasis were treated for 8 weeks with MC903 in one of two different cream bases or with a placebo cream. Biopsies were taken before and after treatment. In addition an uninvolved area of skin was treated during the last 3 weeks and this as well as control areas were then sellotape stripped and biopsied after 8 h. Clinical improvement was seen in eight out of 11 patients treated with MC903 but there was no reduction in the level of ODC in psoriatic lesions after 8 weeks of treatment. The levels of ODC in the tape-stripped uninvolved skin after 3 weeks of treatment with MC903 averaged 22.5 +/- 4.2 pmol/min/mg protein as compared to 58.6 +/- 12.6 pmol/min/mg protein (P = 0.004). The trauma-induced induction of ODC activity was markedly inhibited by the application of MC903.