It has been suggested that one of the mechanisms by which estrogen protects against postmenopausal osteoporosis is by modulating the production of cytokines, such as interleukin 1 (IL 1), tumor necrosis factors (TNF), and interleukin 6 (IL 6), in the bone microenvironment. In this study, reverse transcription/polymerase chain reaction (RT/PCR) was used to analyze expression of the mRNAs encoding these cytokines in freshly isolated human bone biopsy samples. Marked differences were found in the prevalence with which certain cytokines were expressed in different patient subgroups. Specifically, IL 1α, IL 1β, and IL 6 mRNAs were expressed significantly more often in bone samples from postmenopausal women with osteoporotic fractures than in postmenopausal women with normal bone density or postmenopausal women on hormone replacement therapy (HRT). The prevalence of IL 1α expression was 54% in bone samples from patients with osteoporotic fractures (n = 22), compared with 30% in nonosteoporotic postmenopausal patients (n = 10) and 10% in postmenopausal patients on HRT (difference between groups by χ test = 7.0; DF = 2,p < 0.05). Corresponding figures for IL 1β were 54 versus 30 versus 0% (χ = 8.6; DF = 2, p < 0.01) and, for IL 6, 94 versus 51 versus 40%; χ = 13.5; DF = 2, p < 0.01). TNF α was expressed in a similar proportion of osteoporotic (63%) and normal postmenopausal patients (60%), whereas only 10% of HRT treated patients showed expression of TNF α (χ = 8.2; DF = 2, p < 0.05). TNF β was less commonly expressed in osteoporosis (18%) and was not detected at all in normal postmenopausal or HRT treated patients, but this difference was not significant. Cytokine expression in bone samples from young healthy individuals showed sex dependent differences in cytokine expression. In males, the pattern was similar to that observed in normal postmenopausal patients, with IL 1α expression in 36%, IL 1β in 36%, TNF α in 63%, TNF β in 9%, and IL 6 in 54% of cases. In young females (n = 7), however, the patterns of expression were similar to those in the HRT group, with IL 1α expression in 28%, IL 1β in 14%; TNF α in 43%, TNF β in 6%, and IL 6 in 28%. These experiments demonstrate that several cytokine mRNAs are expressed in the bone microenvironment. The high proportion of osteoporotic biopsies that showed expression of IL 1α, IL 1β, TNF α, and IL 6 mRNAs lends support to the hypothesis that activation of cytokine expression in bone is a feature of postmenopausal osteoporosis and raises the possibility that local release of these cytokines contributes to bone loss in this condition.