BACKGROUND Urinary N-telopeptides of type I collagen (NTx) is a specific marker of bone resorption. The NTx levels of children have been reported to be higher than in adults, but such reports are limited. The aim of the present study was to measure the urine NTx of healthy children to define reference values, and study the circadian rhythm of urine NTx excretion between the first and second voided urines. METHODS Three hundred and sixty-two healthy Japanese children (209 boys and 153 girls; age range, 6-11 years) served as subjects to study age-related changes in NTx. Urine samples were collected as the first voided specimen of the day. To evaluate the circadian variation, we measured the excretion of urinary NTx of the first and second voided urine specimens in 30 healthy Japanese children (15 boys and 15 girls), 6-12 years of age. Urine NTx was measured on enzyme-linked immunosorbent assay (ELISA). RESULTS Urinary NTx levels were higher in children of both genders than in healthy adults. In boys, the urinary NTx levels decreased with age. In girls, the urinary NTx levels decreased with age between 6 and 9 years of age, but increased between 9 and 10 years of age. NTx of the first voided urine specimen was significantly higher than the second voided urine specimen. Moreover, a significant linear relationship between the first voided urine and the second void urine existed in both genders. CONCLUSIONS Urinary NTx levels in children are higher than in adults. The urine NTx of the first voided urine specimen may be a good marker of bone resorption for children.