Psoriasis is a skin disease, characterized by the presence of sharply demarcated plaques, typically appears as raised areas of inflamed skin covered with silvery white scaly skin. Its overall prevalence is approximately 2-3%. Although the etiology of psoriasis is not known, genetic, enviromental and immunologic factors appear to influence the susceptibility of the disease. Psoriasis can involve an invalidating state in its more serious shapes, like artropatic, amicrobic pustular and erythrodermic psoriasis. The authors propose a medical-legal classification of the disease, finalized to define of its invalidating degree and a connected practical case to the problem list, often underestimated.