The purpose of this study was to investigate the stability of somatotypes in Belgian children and adolescents, 52 boys and 30 girls, followed longitudinally from 6 to 17 years of age. The anthropometric Heath-Carter somatotypes, with a stature correction for endomorphy, were estimated at 1-year intervals. Mean somatotypes were most different between the earliest and oldest ages in both boys and girls. For boys, the means from 9 to 13 years and from 14 to 17 years did not differ. Means were 2-4-21/2, 21/2-4-4, and 2-4-4 at 6, 12, and 17 years, respectively. The scatter of somatotypes about their means was smallest at 6-8 years and greatest at 11-13 years in both boys and girls (P < 0.05). For girls, the mean somatotypes from 8 or 9 years through adolescence were not different, although they tended to become more meso-endomorphic. Means were 2-41/2-21/2, 3-4-31/2, and 31/2-31/2-3 at 6, 12, and 17 , years, respectively. The average migratory distance for boys was 6.4 (range = 3.7-12.9) and 7.8 (3.9-21.8) for girls. Many subjects had changes that were three to four times greater than others. Boys were lower in ectomorphy at 6-8 years than at older ages, while girls were higher in mesomorphy at 6 than at 12-17 years (P < .05). Interage partial correlations for each component were highest (r(2) greater than or equal to .49) between adjacent years, but were poor to moderate as time intervals increased. The findings of this longitudinal study confirm and further define the instability of somatotypes previously observed in cross-sectional studies of Belgian children and adolescents. (C) 1995 Wiley-Liss, Inc.