Previous research into the origins of lower incisor crowding have not yet established the part played by skeletal morphology in its development. Longitudinal cephalometric records of twelve cases with spaced lower teeth were compared with those of twelve moderately crowded cases and twelve severely crowded cases. Significant differences were found between the three groups. These suggested that crowded cases are more likely to have larger Frankfort--mandibular and occlusal plane angles and shorter posterior face height and mandibular body. Both lower incisors and mandibular symphysis were found to be less protrusive in the crowded cases. During the period of study, from 8 years to 14 years, mandibular prognathism increased more in the spaced cases, although labial inclination of lower incisors increased less in these subjects.