The following case report describes the development of a lingual mandibular bone cavity in a child. Previous cases of these cystlike mandibular defects have been, to the best of my knowledge, all observed in adults. Some of the literature and the different etiologies are reviewed. In this case report the first examination showed normal bone pattern in the mandible, and it was then possible to follow the development of the mandibular bone cavity for about 5 years. During this period the defect expanded until it reached what seems to be a mature stage. The radiographic technique used included lateral, occlusal, and posteroanterior projections.