Hepatocyte growth factor/scatter factor (HGF/SF) has recently been shown to stimulate the hair follicle growth of mouse vibrissae in vitro. In this study, we analyzed the effect of cutaneous injections of recombinant human HGF/SF on hair follicle growth using mice in different hair cycle stages. Five male newborn mice, five male mice in second anagen, and five male mice in second telogen were administered a dorsal intradermal injection of 1 microg HGF/SF dissolved in 0.1% albumin-phosphate-buffered saline once daily for five or seven consecutive days, and then sacrificed on days 7 or 10. Hair follicle growth was evaluated photometrically and histologically using three parameters: the skin color of the reverse side of the resected skin, the skin thickness, and the area occupied by hair follicle tissue. The HGF/SF injected skin of newborn mice had hair follicles that were histologically longer and larger than those of the 0.1% albumin-phosphate-buffered saline injected skin. Mice that had received HGF/SF injection in second anagen, retained anagen hair follicles after 10 d only at the injection site, suggesting that HGF/SF delayed the transition from anagen to telogen. The HGF/SF injected skin of telogen mice had a significant increase in hair follicle tissue in the dermis, suggesting a mild anagen inducible activity by HGF/SF. Furthermore, precise measurements of the 20 hairs plucked from the HGF/SF injection sites revealed mild hair elongation in all the aforementioned experiments. These results imply that HGF/SF acts as a paracrine factor that alters cyclic hair growth of mice.