Abstract In children with acute lymphoblastic leukemia (ALL), the metabolism of type I collagen, the major collagen of bones, may be changed at diagnosis and during early chemotherapy. In the present study, bone formation and degradation rates were evaluated longitudinally in 35 children with ALL, using two serum markers of bone collagen formation: the aminoterminal (PINP) and carboxyterminal (PICP) propeptides; and a marker of degradation: the carboxyterminal telopeptide of type I collagen (ICTP). These serum markers were determined at diagnosis, during induction treatment (at 1, 4, and 6 weeks), and during consolidation treatment (at 8 and 12 weeks). The changes in the serum markers suggested that, at diagnosis, type I collagen turnover (i.e., both synthesis and degradation) was remarkably low. The median serum levels of PINP, PICP, and ICTP were −2.6 SDS (standard deviation score), −1.5 SDS, and −2.5 SDS, respectively. The PICP and PINP levels declined further during the first week of therapy ( p < 0.001), whereas the ICTP levels had risen by end of the induction phase ( p < 0.05). By the end of the 12 week interval, the concentrations of the formation and degradation markers had returned to normal ( p < 0.01). Our findings suggest that ALL is accompanied by low turnover of bone collagen. The abnormalities are at first aggravated, but then corrected, by treatment.