The retinoic acid derivative Ro 10-9359, given orally in maximum daily doses of 100 mg to 15 patients with psoriasis vulgaris, led to clearing of the lesions in 8 patients. The average period of treatment was 9 weeks. In 14 patients side-effects were observed, the most important being palmo-plantar desquamation, shivering, thirst and diffuse hair loss. In 2 patients scaling led to withdrawal. In 6 patients abnormal alkaline phosphatase and glutamic-aspartatic transaminase values, partly transient, developed (glutamic-aspartatic transaminase, p < 0.01).