Nine patients with severe or therapy-resistant psoriasis were treated by plasma exchanges or leukapheresis; one received both treatments in succession. None of the patients showed convincing signs of improvement. We therefore conclude that there is little evidence for the existence of a "psoriasis factor", the removal of which, it has been suggested, would explain the beneficial effects of dialysis. Nor is there anything to indicate that the removal of large numbers of leukocytes would bring about healing.