Conflicting results have been reported in several cross-sectional studies measuring cytokine production from adherent monocytes in pre- and postmenopausal women. Furthermore, the target cells for the action of estrogen are still debated. We therefore assessed in a longitudinal manner the cytokine production from different fractions of peripheral blood mononuclear cells (PBMC) cultured for 48 h. PBMC were obtained from 30 postmenopausal women before and after 6 months of hormone replacement therapy (HRT). Women were randomly allocated to two groups: an adherent PBMC group (n= 20) and a total PBMC group (n= 9). After 6 months of treatment, urinary pyridinoline levels were markedly decreased in both groups (353 ± 24 vs 114 ± 13 μg/mmol creatinine and 325 ± 35 vs 164 ± 31 μg/mmol creatinine respectively, p<0.01). Culture supernatants were assayed for interleukin 1β (IL-1β), interleukin 6 (IL-6), soluble IL-6 receptor (IL-6rs) and tumor necrosis factor alpha (TNF-α). In the adherent PBMC group, HRT induced a nonsignificant trend toward decreased levels of IL-1β (35 ± 10 vs 13 ± 5 pg/ml), TNF-α (333 ± 58 vs 222 ± 30 pg/ml) and IL-6 (115 ± 70 vs 17 ± 10 pg/ml). In contrast, in the total PBMC group, HRT induced a consistent and dramatic decrease in levels of IL-1β (104 ± 22 vs 25 ± 8 pg/ml), IL-6 (5950 ± 1041 vs 1011 ± 361 pg/ml), IL-6rs (148 ± 33 vs 35 ± 12 pg/ml) (p<0.01) and TNF-α (1468 ± 315 vs 585 ± 207 pg/ml, p= 0.05). We then evaluated whether HRT had the same effect in vitro. Adherent or total PBMC of 8 postmenopausal women were cultured with or without 10⁻⁸M 17β-estradiol or tibolone for 48 h. Production of IL-1β, TNF-α, IL-6 and IL-6rs was not affected by the presence of 17β-estradiol or tibolone in cultures of these cell fractions. In conclusion, our data indicate that non-adherent PBMC could mediate the response to HRT. HRT may exert its action indirectly via noncirculating cells, as suggested by the absence of an in vitro effect.