Serum (S-) or urinary (U-) type I collagen N-telopeptide (NTX) reflect bone resorption by cathepsin K or matrix metalloproteinase (MMP). In our analysis of 18 patients with serum creatinine (S-Cr) less than 2.0 mg/dL, there was a significant positive correlation between S-NTX and U-NTX (U-NTX = 3.7 x S-NTX-13 [r = 0.84, P<0.0001) ] . Because diurnal and day-to-day variations are large in U-NTX assay, it is recommended that U-NTX be measured in the first or second urine of the day. In contrast, physiological variation in S-NTX is relatively small. However, we should bear in mind that S-NTX is interfered with renal function. Before interpretation of S-NTX, we should estimate glomerular filtration rate by using Cockcroft-Gault formula, especially in elderly patients with low body mass index such as rheumatic patients, whose S-Cr-is in normal ranges Both markers are very useful for estimating the risk of bone fracture and for monitoring therapeutic effects of anti-resorptive agents such as bisphosphonate or selective estrogen receptor modulator.