Vitamin D is recognized as a prohormone that has two forms: vitamin D2 (ergocalciferol) and vitamin D3 (cholecalciferol) having 1,25 (OH) 2-vitamin D as active form. The circulating serum level is mainly 25 (OH) - vitamin D which is considered deficient at values lower than 20ng / ml. Due to the identification of its receptors in various cells, its immunomodulatory role is nowadays investigated, in the prevention and treatment of neoplasms, in autoimmune and cardiovascular diseases. It is of great interest to establish the evolution of serum values of 25 (OH)- vitamin D in relation to systemic therapy for vulgar psoriasis given the multiple implications of 25 (OH) - vitamin D in the pathogenesis of psoriasis both in keratinization and immunological process of dendritic cells and T lymphocytes.