Psoriasis is a chronic inflammatory skin condition, characterized by T-helper 
(Th) 1 and Th17 cell activation. Ustekinumab is a fully human immunoglobulin G1κ 
monoclonal antibody that targets the common p40 subunit that is shared by both 
interleukin (IL)-12 and IL-23, consequently inhibiting T-cell differentiation 
along both Th1 and Th17 pathways. This is a report of two patients who developed 
psoriatic arthritis during ustekinumab treatment for psoriasis. Neither patient 
had a personal or family history of arthritis.