To analyze the inhibitory effect of hormone replacement therapy (HRT) on bone resorption, we measured urinary excretion of C-telopeptide (CTX) and N-telopeptide (NTX) of type I collagen as new markers of bone resorption and assessed their correlation with bone mineral density, in comparison with urinary pyridinoline (Pyr), deoxypyridinoline (D-Pyr) and hydoxyproline (Hpr). CTX and NTX, in addition to Pyr, D-Pyr, and Hpr, in urinary samples from 33 postmenopausal women with climacteric symptoms who were treated with 0.625 mg conjugate equine estrogen and 2.5 mg medroxyprogesterone acetate for 12 months were measured using ELISA for each telopeptide. Bone mineral density in the lumbar spine was also measured by dual energy X-ray absorptiometry. Similar to Pyr, D-Pyr, and Hpr, urinary excretions of CTX and NTX significantly decreased during the continuous administration of conjugate equine estrogen and medroxyprogesterone acetate for 12 months. The magnitudes of the reduction of CTX and NTX were significantly greater than those of Pyr, D-Pyr, and Hpr. CTX and NTX correlated well with Pyr, D-Pyr, and Hpr, and there was a good correlation between CTX and NTX. Unlike Pyr, D-Pyr, and Hpr, the pretreatment values of CTX and NTX correlated negatively with bone mineral density at baseline. These results suggest that, among the markers for bone resorption, the changes in CTX and NTX could be greater than those in Pyr, D-Pyr, and Hpr during HRT in postmenopausal women.