We report a new case of aggressive juvenile fibromatosis (A.J.F.) in a 20-month-old girl. The lesion affected the inferior border of the left mandible and the adjacent soft tissues. The child presented with a painless mass, which had grown over a period of 2 months. Radiographs and computed tomographic scan showed a multilacunar bone defect with subcutaneous and gingival involvement. A biopsy was performed, followed by a partial hemimandibulectomy and a costal graft. The surgical specimen measured 4 X 3 X 3 cm. The patient did well 1 year after surgery. 16 cases of A.J.F. have been reported; with ours, 14 are mandibular. A.J.F. is a locally aggressive lesion, which doesn't metastasize. It occurs chiefly in childhood and adolescence from 1 1/2 to 18 years (median 6.5 year-old). Duration of symptoms prior to presentation varies from a few weeks to months. Clinically, it is a firm nodule. Radiographs are non-specific, but 9 cases have poorly defined destruction of the mandibular inferior border. Following a block resection of the tumor (13 cases), there is no recurrence. In this review, we discuss the clinico-pathologic diagnosis of this impressive tumor which is compared with other mandibular fibrous tumors in children.