Immunodeficiency due to inborn errors, infections, autoimmune diseases and the toxicity of certain therapies, has become an important clinical challenge. Strategies to enhance immune function could improve the outcome for a considerable number of patients. Although myeloid growth factors have found widespread use in clinical medicine, a ‘lymphoid growth factor' has not yet been established in clinical use. Several recent studies have proposed two candidate immunostimulatory cytokines: interleukin-7 (IL-7) and IL-15. In this Review, we discuss recent findings regarding the effects of IL-7 and IL-15 on lymphopoiesis and their potential use for the treatment of immunodeficiency.