Methimazole, an antithyroid drug, was orally administered, in an open trial, in a dose of 20 mg every 12 h for 8 weeks to 8 volunteers with long-standing psoriasis. 3-mm punch biopsies were taken from the lesions at the start and at the end of the study. Clinical response was assessed using the Psoriasis Areas Severity Index score. Methimazole produced marked to moderate improvement in the clinical scores in the majority of patients. Histological scores were also significantly improved in all patients. Unexpectedly, thyroid function tests were not affected by methimazole therapy in all but one patient, and none of the patients developed drug-induced cytopenia. Methimazole may be an effective therapeutic agent in the management of psoriasis; it most probably exerts its therapeutic effect by acting on the immune system.