Psoriasis is a common, chronic, inflammatory, multisystem disease with predominantly skin and joint manifestations affecting approximately 2% of the population. In this fourth of 6 sections of the guidelines of care for psoriasis, we discuss the use of traditional systemic medications for the treatment of patients with psoriasis. Treatment should be tailored to meet individual patients' needs. We will discuss in detail the efficacy and safety, and offer recommendations for the use of the 3 most commonly used, and approved, traditional systemic agents: methotrexate, cyclosporine, and acitretin. We will also briefly discuss the available data for the use of azathioprine, fumaric acid esters, hydroxyurea, leflunomide, mycophenolate mofetil, sulfasalazine, tacrolimus, and 6-thioguanine in psoriasis. © 2009 American Academy of Dermatology, Inc.