Interleukin-1 (IL-1), a cytokine best known for its ability to stimulate lymphocyte proliferation, has recently been shown to stimulate bone resorption and modulate bone formation in vivo. Consequently, the authors have devised a series of studies to investigate the relationship between bone remodeling, menopause, and monocyte IL-1-secretion. In a first study, monocytes from osteoporotic patients were found to produce more IL-1 than monocytes from control subjects. IL-1 activity was also found to reflect histomorphometric indices of bone formation, but not of bone resorption. In a second study, devised to assess the effect of menopause on the relationship between IL-1 and bone turnover, a significant correlation was found between IL-1 and BGP in premenopausal osteoporotic women and osteoporotic men, but not in both postmenopausal osteoporotic subjects and normal subjects of either sex. In a third study, IL-1 from untreated postmenopausal women was found to be higher than in either untreated premenopausal or estrogen/progesterone-treated postmenopausal women. A significant negative correlation was found between IL-1 and years since menopause in both the healthy and osteoporotic postmenopausal women. Premenopausal IL-1 levels were achieved within eight years of menopause in the healthy but not in the osteoporotic subjects. In osteoporotic women, high IL-1 levels were evident as long as 15 years after menopause. IL-1 also correlated inversely with mineral density as measured by quantitative computer tomography. In prospective study, treatment with estrogen/progesterone caused a significant increase in IL-1 activity. This data indicates that monocyte IL-1 production mirrors the rate of bone turnover in both the healthy and osteoporotic patient, and that alteration in IL-1 production may underlie the postmenopausal acceleration of bone loss and its inhibition by ovarian steroids.