Dermatitis occurring as a side effect in psoriatic patients during oral administration of the retinoid acid derivative Ro 10-9359 is described. This so-called retinoid dermatitis exhibits a characteristic disseminated pattern. Sites of predilection are the face, the exterior surface of the upper and the interior surface of the lower arms, the superior thoracic aperture, the back of the hands and the flanks. The lesions present as follicular papules and/or vesicles. The histological picture is that of acute non-specific dermatitis. This retinoid dermatitis was observed in 9 our of 23 patients (39%) treated with Ro 10-9359. Other side effect such as erythema, desquamation, itching and, rarely, a burning sensation showed the same distribution. The characteristic dermatitis, as well as the other side effects mentioned, occur dose-dependently within the normal therapeutic range of Ro 10-9359 for psoriasis (0.5--1 mg/kg bodyweight daily).