Ninety-seven patients with severe psoriasis took part in a 1-year study to evaluate the effect of a new oral synthetic retinoid (Ro 10-9359). The trial was performed in a double-blind cross-over fashion. The treatment started with either 100 mg daily of Ro10-9359 or placebo and the maintenance dose was in most cases 50 mg. Follow-up examinations were performed monthly and the parameters erythema, desquamation, infiltration and extent of the lesions were followed. Throughout the study there was a significant to highly significant preference for Ro 10-9359 shown by all parameters. More patients were in complete remission after Ro 10-9359 periods than after placebo periods. The side-effects of Ro 10-9359 on uninvolved skin and mucous membranes seemed to be largely dose-dependent. Twenty-three patients interrupted the study, four of them because of side-effects, mainly alopecia. Laboratory examinations revealed no aberrations which could be attributed to the therapy. One patient developed hepatitis during a placebo period.