Background: This study aimed to investigate immunoregulatory role of IL-2/STAT5/CD4+CD25+Foxp3 Treg pathway in pathogenesis of chronic osteomyelitis (COM). Methods: Sprague-Dawley (SD) rats were injected with Staphylococcus aureus to establish COM model. 4 weeks later, the lesioned bones were collected and subjected to HE staining for examination of inflammatory infiltration. Enzyme-linked immunosorbent assay (ELISA) was employed to detect IL-2 expression in peripheral blood; flow cytometry was performed to detect CD25+CD4+Foxp3 Treg cells in peripheral blood. The mRNA expression of Foxp3 and CTLA-4 was detected by RT-PCR and the protein expression of STAT5 and p-STAT5 was detected by Western Blotting in CD25+CD4+Foxp3 Treg cells. Results: In COM group, the periosteal thickening was observed in femur, and there were a large number of inflammatory cells in medullary cavity, accompanied by bone destruction. At 1, 2 and 4 weeks, IL-2 expression significantly increased, the proportion of CD4+CD25+FoxP3 Treg cells in peripheral monocytes markedly increased, the mRNA expression of Foxp3 and CTLA-4 and p-STAT5 protein expression increased dramatically in Treg cells as compared to control group (P<0.001). Conclusions: IL-2/STAT5/CD4+CD25+Foxp3 Treg pathway may be involved in the pathogenesis of COM, and excessive immunosuppression may lead to persistent infectious inflammation, which may become a key target for future treatment of COM.