The relevance of salicylic acid in dithranol creams was evaluated in a double-blind study. Patients with chronic plaque psoriasis were treated using a short-contact schedule for dithranol on an outpatient basis. A left-right comparison was carried out between sites treated with either dithranol with 2 % salicylic acid (D+S) or dithranol in the same base without salicylic acid (D-S). Clinical results were evaluated once a week using the psoriasis area severity index. In order to quantify the improvement, flow cytometric measurements were done using the monoclonal antibody K_s8.12, recognizing keratin 16 in normal and lesional epidermis. Simultaneously, relative DNA content was quantified which previously was described as a useful method to monitor a therapeutic effect. Both PASI scores and K_s8.12 binding decreased after 6 weeks treatment with D+S and D-S. However, percentages of cells in SG₂M phases did not show a significant change. No significant difference was observed between sites treated with either D+S or D-S. Therefore we conclude that the addition of salicylic acid in a concentration of 2 % does not enhance the efficacy of dithranol creams and we confirm that K_s8.12 is a useful quantitative marker for therapeutic efficacy.