Thirty-nine patients with psoriasis (12 females, 27 males) entered a randomised, double-blind, placebo-controlled study on the efficacy of fumaric acid therapy in an outpatient setting. During 16 weeks the patients were treated with tablets containing a combination of dimethylfumarate and different salts of monoethylfumarate, with octylhydrogen fumarate or with placebo tablets. All patients were treated with identical indifferent topical therapy and followed an elimination diet (avoidance of spices, wine and nuts). Thirty-four patients completed the study. Five patients dropped out because of side effects or aggravation of the skin lesions. The patients treated with the combination of monoethyl- and dimethylfumarate showed a significantly better therapeutic response compared with those who were treated with placebo or octylhydrogen fumarate. Side effects of the fumarate containing tablets were flushing, diarrhoea, a reversible elevation of transaminases, lymphocytopenia and eosinophilia. One patient developed a disturbance of the kidney function which normalised after discontinuation of the therapy.