In order to study the function and the level of activity of the mentalis and the orbicularis oris superior muscles in subjects with different lip morphology, an electromyographic investigation was carried out on ten children with normal lip position (competent lips) and ten children with lip incompetence. The underlying skeletal and dento-alveolar morphology was studied by cephalometrics. All the children had malocclusions, but had received no orthodontic treatment. It was found that a relaxed lip position with no EMG activity could be taken up by all the children in both groups. Lip closure, mastication, and swallowing were performed with a significantly greater activity in subjects exhibiting lip incompetence than in those with normal lip position. The analysis of cephalometric values in relation to lip posture and perioral muscle activity showed that subjects with incompetent lips and increased perioral muscle activity had great lower anterior face height, high mandibular plane angle, postnormal skeletal jaw relation, and increased lower incisor proclination.