In this study, we investigated hepatic fibrogenesis caused by long-term thioacetamide (TAA) administration in ob/ob mice, a naturally occurring leptin deficient animal. In the lean littermates, prominent hepatic fibrosis, as well as positive staining for α smooth muscle actin (α-SMA), was induced by treatment with TAA (200 μg/g, IP, 3 times per week) for 4 to 8 weeks as expected. In sharp contrast, almost no hepatic fibrosis developed in ob/ob mice given the equivalent doses of TAA, where specific staining for α-SMA barely was detected. Induction of α1(I) procollagen mRNA caused by TAA also was prevented in ob/ob mice almost completely. Further, transforming growth factor β (TGF-β) mRNA was increased in the liver after TAA treatment for 4 weeks in lean littermates, which also was prevented in ob/ob mice. Interestingly, fibrotic septa in the hepatic lobules, as well as increases in α1(I) procollagen mRNA, was observed in ob/ob mice, when they were injected with recombinant murine leptin (1 μg/g daily) in combination with TAA treatment. Leptin per se did not cause any fibrotic changes in the liver in ob/ob mice. These findings clearly indicated that leptin deficiency is responsible for the resistance to TAA-induced profibrogenic responses in ob/ob mice. In conclusion, leptin appears to promote profibrogenic responses in the liver, in part, by up-regulation of TGF-β.