Craniofacial growth was followed longitudinally in two groups of boys with low and high MP-SN angles. The purpose was, first, to reveal group differences in dimensional change, and second, to find whether such differences were associated with a group difference in mandibular growth rotation. Group differences in dimensional change were explained by a difference in matrix rotation of the mandibular corpus, especially in the 6- to 12-year period. In the 12- to 15-year period, matrix rotation was similar in the two groups and so were dimensional changes. Morphologically, dimensional group differences in the 6- to 12-year period were theoretically compatible with the fact that mandibular rotation was clearly more forward in the low angle than in the high angle group. Statistically, dimensional variables with significant group difference were correlated strongly with matrix rotation and, in most cases, non-significantly with intramatrix rotation.