IL-20 is a member of the IL-10 family of cytokines that also includes IL-10, , and IL-26 as well as a number of viral homologs. Keratinocytes and monocytes in states of activation can produce IL-20. It transmits signals via two different cell-surface receptor complexes resulting in the activation of STAT3 in keratinocytes and other epithelial cells. Thus it appears to be primarily an inducible autocrine and paracrine factor that regulates proliferation and differentiation of keratinocytes in the context of inflammation. Several findings indicate that IL-20 mediates the hyperproliferation of keratinocytes associated with cutaneous inflammation. As such it is likely to have a central role in inflammatory skin diseases such as psoriasis and eczema. IL-20 also promotes the expansion of multipotential hematopoietic progenitor cells indicating a role beyond the response of epithelial cells to inflammation. Increased understanding of the biology of IL-20 may provide novel opportunities for diagnostic methods and therapeutic intervention.