Low-dose IL-2 therapy was well tolerated and showed indications of clinical efficacy in a phase I–IIa trial of 46 patients with any one of 11 autoimmune diseases (including rheumatoid arthritis, ankylosing spondylitis, systemic lupus erythematosus and several forms of vasculitis). Patients received low-dose IL-2 (1,000,000 IU/day) for 5 days, then once every 2 weeks for 6 months. Across all diseases, regulatory T cells, but not effector T cells, were expanded and activated following treatment, indicating a potential use for low-dose IL-2 therapy across the spectrum of autoimmune diseases.