Urinary excretion of type I collagen cross-linked N-telopeptides was studied in 52 children and adolescents with osteogenesis imperfecta (OI) and found to be above the 75th percentile of controls in 44 of the patients. OI patients suffering from fractures during the preceding 6 months had significantly higher values (p < 0.05). In contrast, patients with better motor performance tended to have lower values (p = 0.059). The concentration of urinary type I collagen cross-linked N-telopeptides was positively correlated with urinary calcium excretion (p < 0.05), which was found to be elevated in 20 of the patients. Our results show that during childhood and adolescence in OI not only the synthesis but also the turnover of mature cross-linked type I collagen is disturbed and provide evidence that bone resorption rates are elevated.