The medical records of 241 children with stage II osteosarcoma were reviewed to assess whether growth characteristics at the time of diagnosis are related to treatment outcome. We defined the maximal growth rate period as an age between 12 and 14 for girls and between 13 and 15 for boys. Accordingly, we subdivided patients into 3 groups and analyzed for clinical characteristics and metastasis-free survival (MFS). Most of the clinical characteristics showed no difference among the 3 groups. After a median follow-up of 54 months (range: 6 to 153 mo), the 5-year MFS of the entire cohort was 67.5%±3.1%. Patients in the period of maximal growth rate did worst (57.0%±5.6%, P=0.02), whereas patients before and after the period of maximal growth rate fared better (77.0%±5.1% and 69.4%±4.9%, respectively). On multivariate analysis, disease occurrence in the period of maximal growth rate [relative risk (RR) 2.44; 95% confidence interval (CI) 1.37-4.38; P=0.003], American Joint Committee on Cancer stage IIB disease (RR 2.49; 95% CI 1.49-4.17; P=0.001), and a poor histologic response (RR 2.91; 95% CI 1.78-4.77; P<0.001) independently shortened the MFS. The growth characteristics of patients that are available at the time of diagnosis carry prognostic significance and these could be used as a base to design risk-adapted therapy for osteosarcoma.