Spinal mobility and posture were measured and their correlations with growth velocity were calculated in 30 boys and 30 girls aged 13 to 14. The spinal measurements were carried out by noninvasive goniometric methods. In the thoracic spine kyphosis (P less than 0.01), forward flexion (P less than 0.01) and the sum of lateral flexions (P less than 0.05) were reduced in the girls compared with the boys. In the boys and girls alike, thoracic rotation to the left was smaller than to the right, but the difference was statistically significant (P less than 0.05) only for the girls. In the girls, thoracic forward flexion and rotation to the left had negative correlations (r = -0.38 and -0.39, P less than 0.05) with growth velocity. The hypothetical significance of the results for the explanation of the development of adolescent idiopathic scoliosis is discussed.