Background Receptor activator of nuclear factor kappa B (NF-κB) ligand (RANKL) and osteoprotegerin (OPG) have been recently shown to play important roles in bone resorption. The aim of this study was to investigate the possible association between the expression of bone resorption regulators (RANKL and OPG) and inflammatory cell infiltration in chronic apical periodontitis.Methods The samples of chronic periapical lesions (n=40) and healthy periapical tissues (n=10) were examined for immunohistochemical analysis of RANKL and OPG. Lesion samples were further analyzed for the inflammatory infiltration condition. The inflammatory cell infiltration was scored in relation to immunohistochemical reactivity for CD3, CD20 and CD68.Results The number of RANKL-positive cells and the ratio of RANKL/OPG in chronic apical periodontitis were significantly higher than those in healthy periapical tissues (P＜0.001). The number of RANKL-positive cells was higher in lesions with severe inflammatory infiltration than in those with light inflammatory infiltration (P＜0.05). Significantly increased RANKL expression was found with T lymphocytes (CD3+), macrophages (CD68+) and B lymphocytes (CD20+)infiltration (P＜0.05). No association was found between the ratio of RANKL/OPG and inflammatory cell infiltration.Conclusions RANKL expression was increased with T, B lymphocytes and macrophages infiltration, respectively in chronic periapical lesions. RANKL appears to be closely related to periapical inflammatory infiltrates. The relative ratio of RANKL/OPG may be a key determinant of RANKL-mediated bone resorption.