Eight children in the late mixed dentition stage, with mandibular crowding, increased overbite and overjet, and, on the basis of cephalometric analysis, an expected anterior rotation type of mandibular growth pattern, were treated with a mandibular lipbumper and bite plate for seven months. The present study reports changes in muscle activity. Electromyographical analysis of the activity of the anterior temporalis muscle, the right upper lip and the right and left lower lip revealed no consistent relationship between muscular pattern and the variables expressing occlusal and skeletal morphology. Similarities in occlusion and facial morphology were not directly related to similarities in functional pattern. When changes in the muscle activity in individual patients were related to the basal level of muscular activity, the original morphology of the facial skeleton and the changes during treatment, there was a pattern. Two extreme cases that demonstrate the form/function equilibrium are described. The results of the present analysis suggest that the conventional variables in cephalometric analysis are not appropriate to the study of the form/function interaction.