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 Ks8.12, 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 Ks8.12 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 Ks8.12 is a useful quantitative marker for therapeutic efficacy.