[en] Psoriasis is a chronic relapsing immunoinflammatory dermatosis that is commonly associated with systemic comorbidities. The pathogenic importance of interleukin (IL)-12 and IL-23 is beyond doubt, as well as the involvement of T helper cells (Th)1 and Th17 cells. There is upregulation of the p40 subunit shared by IL-12 and IL-23 and of the IL-23 p19 subunit, but not an increased expression of the IL-12 p35 subunit. This indicates that IL-23 appears more involved than IL-12 in the pathogenesis of psoriatic plaques. Ustekinumab is a fully human monoclonal antibody of the immunoglobulin (Ig) G1 class targeting the p40 subunit common to both IL-12 and IL-23, thus inhibiting both IL-12 and IL-23 receptor-mediated signalling. Ustekinumab is part of the recent biologic therapies active in psoriasis, autoimmune arthritides, and inflammatory bowel diseases.