Estrogen replacement therapy (ERT) prevents bone loss and fracture in early postmenopausal women, but its benefit for women over 70 yr of age has not been determined. We have examined the effect of a short course of ERT on biochemical markers of bone turnover in older women. Eleven women (mean age, 77 yr) were given conjugated estrogen (Premarin; 0.625 mg/day) for 6 weeks. Biochemical markers were measured on serum and urine collected at baseline (two samples), after 5 and 6 weeks of ERT, and 5 and 6 weeks post-ERT. Markers of bone formation were osteocalcin, bone alkaline phosphatase, and type I procollagen peptide. Markers of bone resorption were total urinary hydroxyproline, total and free pyridinoline and deoxypyridinoline cross-links, type I collagen cross-linked N-telopeptides, and serum C-terminal cross-linked telopeptide. Data were analyzed by repeated measures multivariate analysis of variance to estimate the overall effect of ERT on the biochemical markers. Markers of bone resorption decreased during ERT and returned to baseline after ERT (P < 0.05). Markers of bone formation declined less during ERT and continued to decline after ERT (P < 0.05). We conclude that ERT reduces bone turnover in older women and that markers of bone turnover may be useful in assessing the response to treatment in this age group.