Skins biopsies from psoriatic skin and blood samples will be taken before (day 0) and at several time points after start (day 4, week 2-3, and week 6-7) of dithranol therapy to be subjected to an explorative analysis (see outcome measures). A total of maximal 15 psoriasis patients will be enrolled. Normal lesion-adjacent skin from 10 patients undergoing surgery for non-inflammatory conditions will serve as control.