INTRODUCTION: Coinciding treatment with periods of accelerated skeletal growth and maturation might be advantageous in clinical practice. Better understanding of the concordance between skeletal and chronologic ages during the period that children frequently receive orthodontic treatment is needed. The literature on skeletal age determination from hand-wrist radiographs lacks reports based on longitudinal data, creating lacunae in the understanding of the magnitudes and variations of differences between skeletal and chronologic ages. The aims of this research were to comprehensively analyze the concordance between skeletal and chronologic ages determined by using the Greulich and Pyle method at different ages in the preadolescent and adolescent periods, and to determine any age- and sex-related differences in the concordance. METHODS: By using the Greulich and Pyle method, skeletal age determinations were made from 572 hand-wrist radiographs of 68 white children with normal facial growth, selected from the records of the Burlington Growth Centre, spanning the growth period from 9 to 18 years. Skeletal age and chronologic age differences for each sex were analyzed by using paired t tests and Wilcoxon signed rank tests at yearly intervals. Differences over the longitudinal duration were examined by using the mixed model approach. The limits of agreement were determined by using the Bland-Altman method. In each yearly chronologic age group, differences were clinically categorized based on the proximity of the skeletal and chronologic ages. RESULTS: Overall, a slightly greater proportion of the total skeletal age determinations made in girls (41.9%) were within 0.5 year of their chronologic age, compared with 38% in boys. The largest proportions of subjects having skeletal age-chronologic age differences within 0.5 year were in the 10-year age group in girls (64.5%) and the 13-year age group in boys (64.7%). Mean skeletal age-chronologic age differences were significantly larger in the 13- to 16-year age groups in girls and in the 16- and 17-year age groups in boys, but the differences were not statistically significant at other ages. Several patterns of variations were identified in the direction of differences when individual plots were examined. CONCLUSIONS: This longitudinal analysis of differences between skeletal and chronologic ages showed wide ranges and distributions of differences at each yearly age group during the growth period from 9 to 18 years, even when mean differences were small. Variations in the magnitude and direction of differences observed at different ages highlighted the variability in skeletal maturation among normally growing young people. Overall, the differences in skeletal and chronologic ages were positively related to age, with little effect of sex or its interaction with age.