Objectives: To determine the ranges of variation of circulating receptor activator of nuclear factor-κB ligand (RANKL /osteoprotegerin (OPG)/macrophage-colony stimulating factor (M-CSF) and to ascertain their potential relationships with age, sex and menopausal status in women, and with sex hormones in a population-based healthy cohort. Subjects and methods: Blood samples were collected with EDTA after an overnight fast. The plasma levels of each of the above biochemical indices were measured by ELISA in a total of 566 apparently healthy individuals aged 18-75 years. Results: The plasma concentrations of cytokine molecules in the entire sample ranged from 674 to 4929 pg/ml for OPG, from 105 to 4468 pg/ml for soluble RANKL (sRANKL), and from 187 to 7604 pg/ml for M-CSF. The OPG levels demonstrated a clear positive correlation with age in both sexes (r = 0.42 and 0.43, P < 0.001, for men and women respectively). Application of the two-interval mathematical model revealed that in females OPG levels were age-independent until age 42, but then showed clear and significant correlation with age (r = 0.48, P < 0.001). As a result, young females (before 42 years) had a substantially lower average OPG level, 1377.8±327.68 pg/ml, in comparison with older women, 1666.02±397.14 pg/ml. The M-CSF correlation with age was significantly greater in women (r = 0.29, P < 0.001) compared with men (r = 0.17, P < 0.01). Significant negative correlations between plasma levels of both OPG and M-CSF with estradiol concentrations were observed in women (r = -0.39, P< 0.01; r= -0.25, P < 0.001 respectively). sRANKL did not correlate with either age or sex hormones in either women or men. Conclusion: Age and sex affect differently the interindividual variation of OPG, RANKL and M-CSF. Our observations could form the basis for further research to establish provisional reference limits for OPG and RANKL, which are potential markers for benign and malignant processes in bone.