Syndecan-1 is the prototypic member of a family of heparan sulfate-bearing cell surface proteoglycans that function in adhesion, cell - extracellular matrix interactions, migration, and proliferation. During embryogenesis, syndecan-1 expression in the epithelium is downregulated when the epithelium gives rise to motile mesenchymal cells, whereas mesenchymal syndecan-1 expression is upregulated during organ formation. In aggressive basal cell carcinomas, syndecan-1 expression is evident in the stroma. Some neoplastic cells induce stroma to meet needs for growth, and it may be the mesenchymal cells that produce and shed syndecan-1 into the stroma. The physiologic mechanism by which the hair follicle undergoes its cyclic process of involution and formation of a new active hair follicle is not well understood. Sixty scalp biopsies and a large scalp resection were evaluated for syndecan-1 expression within hair follicles in the growing (anagen), involuting (catagen), and resting (telogen) phases. Strong syndecan-1 immunoreactivity was evident in the outer root sheath (ORS) of the anagen hair follicle, but this expression diminished in intensity with the involution and resting stages in the hair follicle cycle. The diminution of syndecan-1 immunoreactivity in the ORS of involuting and resting hair follicles may be a result of terminal keratinocyte differentiation. Syndecan-1 was also present in the dermal papilla of the anagen hair follicle, where it may promote growth factor-mediated cell signaling that induces and maintains growth of the hair shaft and the inner root sheath.