Interleukin 7 (IL-7) was originally identified as a growth factor for B cell progenitors, and subsequently has been shown to exert proliferative effects on T cell progenitors and mature peripheral T cells as well. Constitutive IL-7 mRNA expression so far had been demonstrated in bone marrow stromal cell lines, thymus, spleen, and among nonlymphoid tissues in liver and kidney. Here we show that both murine and human keratinocytes express IL-7 mRNA and release IL-7 protein in biologically relevant amounts. The physiological or pathological relevance of keratinocyte-derived IL-7 is presently unknown. Our finding that keratinocytes can produce IL-7 in concert with reports that IL-7 is a growth factor for in vivo primed antigen- specific T cells, as well as for T lymphoma cells suggests, however, that keratinocyte-derived IL-7 is important in the pathogenesis of inflammatory skin diseases and cutaneous T cell lymphoma.