Interleukin 6 (IL-6) is an important mediator of hepatocyte proliferation after hepatectomy. However, elevated IL-6 levels are found in patients with chronic liver disease. Therefore, it is unclear if hyperstimulation with IL-6 may have an influence on liver regeneration. We investigated whether a strong activation of IL-6-dependent pathways may change the course of hepatocyte proliferation after hepatectomy. Transgenic mice overexpressing the human soluble IL-6 receptor/gp80 (hsgp80) in hepatocytes were stimulated with or without hepatectomy with human IL-6 (hIL-6). Nuclear extracts were prepared and activation of gp130-dependent pathways was studied by Western blot and gel shift experiments. Cell cycle progression of hepatocytes after hepatectomy was investigated by monitoring cell cycle-specific factors. hIL-6 strongly activates Stat3 for more than 48 hours in human soluble hsgp80 transgenic mice. In contrast, no major differences were evident in the regulation of the Ras/MAP kinase pathway compared with wild-type (wt) mice. Also when hsgp80 mice were stimulated with hIL-6 3 hours before hepatectomy Stat3 is activated for more than 72 hours, whereas in unstimulated mice this event is restricted to the early hours. Strong activation of Stat3 resulted in a delay and inhibition of hepatocyte proliferation as measured by 5-bromo-2'-deoxyuridine (BrdU) staining and Cyclin A and E expression. This observation directly correlates with the induction of the cell cycle inhibitor p21. In summary, strong IL-6-dependent activation of Stat3 before hepatectomy results in delay and inhibition of cell cycle progression after hepatectomy. Therefore our results suggest that hyperstimulation with IL-6 can inhibit liver regeneration.