Estrogen replacement therapy (ERT) is known to prevent bone loss following the menopause, but the mechanism for this is unclear. Estrogen may suppress the secretion of certain bone-resorbing cytokines. The aim of this study was to assess the effect of ERT on the levels of cytokines measured in peripheral blood. We measured cytokines in 10 postmenopausal women (ages 56-59, 3-9 years since menopause) treated with ERT and 10 age-matched (54-59 years, 4-10 years since menopause) untreated women as controls. Samples of blood were taken and used for mononuclear cell cultures, whole blood (WB) cultures, and the separation of serum. The cultures were treated with lipopolysaccharide (LPS; 500 ng/ml) and hydrocortisone (10−6 M). The conditioned medium from cultures and the serum were then assayed for interleukin-6 (IL-6), IL-1α, IL-1β, IL-1 IL-1ra, tumor necrosis factor alpha (TNF-α), and granulocyte macrophage colony stimulating factor (GM-CSF) by enzyme-linked immunosorbent assay. M-CSF and the soluble cytokine receptors soluble IL-6 receptor (sIL-6r) and soluble TNF receptor type 1 (sTNFr1) were also measured in serum and M-CSF in stimulated WB cultures. Measurements were corrected for mononuclear cell count. We also measured serum bone-specific alkaline phosphatase (ibAP) in all subjects. We found that LPS stimulated secretion of all cytokines both in WB and isolated cell cultures, and that this was attenuated by hydrocortisone. A significantly higher ratio of IL-1β/IL-1ra (p = 0.02) in LPS stimulated WB cultures was seen in the untreated women. Levels of IL-1β and IL-1α measured in WB cultures were lower and IL-1ra was higher in the ERT-treated group but these results were not significant. BAP was higher in the untreated group (p = 0.005) and correlated with IL-α/IL-1ra in the whole group (r = 0.49, p = 0.03). Results of other measurements showed no significant differences between groups. We conclude that estrogen may prevent bone loss following the menopause by altering the balance between IL-1β and IL-1ra.