Unilateral absence of the ramus of the mandible is a rare deformity. It represents a clear-cut defect, which has been described under a variety of titles, thus making it difficult to discover in the literature. It is very often associated with microtia (in four out of the five cases here presented) and at times with unilateral fissure of the mouth (in two of the present group of cases).

Embryologically, this defect is thought to be caused by improper development of the various elements of the first branchial arch and cleft. Other facial and cranial bone anomalies, however, are present rather often, especially in the temporal bone. The question may be raised whether these anomalies are not secondary to the absence of the mandibular segment, which thus deprives the surrounding tissues of a normal growth stimulus.

The five cases reported here have been collected over a period of the last eight years. Their treatment in general has been conservative, except in the case of the soft-tissue defects. The repair of mouth fissures and the building up of the ear when absent has not been delayed. The treatment of the mandibular defect is best postponed until the permanent teeth have erupted to permit the use of proper splinting and for other considerations outlined in the last case. Orthodontic measures, however, should be instituted early.