Ro 10-9359 is a retinoic acid derivative, selected for study because of a better tolerance than retinoic acid, shown in animal experiments. Doses of 25 mg b.i.d., 25 mg t.i.d. and 50 mg b.i.d. were administered orally to 27 patients suffering from severe chronic generalized psoriasis. The clinical efficacy was evaluated by means of a new index, psoriasis area and severity index (PASI) based on severity and area of psoriatic lesions. At doses of 25 mg t.i.d. or 50 mg b.i.d. Ro 10--9359 proved to be an extremely potent antipsoriatic drug. A more than 90% reduction of psoriatic lesions could be seen in 10 patients out of 20 after 4-8 weeks of treatment. This good effect lasted about 5 weeks after treatment. Side effects were frequent, but mostly mild and completely reversible after termination of treatment.