Hereditary gingival fibromatosis (HGF) is a rare autosomal dominant disorder characterized by slowly progressive fibrous overgrowth of the gingiva. We report the clinical results of long-term follow-up after gingivectomy in a patient with HGF. A 13-year-old boy was referred to our hospital because of gingival overgrowth. The upper and lower gingiva were severely overgrown. The primary dentition was initially affected, and generalized gum overgrowth caused unerupted permanent teeth. There was no sign of syndrome. His father and two sisters had a similar gingival condition. On the basis of these findings, we diagnosed the lesion as a hereditary gingival overgrowth. A gingivectomy was carried out with the patient under local anesthesia, and there was no recurrence after surgery. Recently, son of sevenless-1 (SOS1) has been reported as a responsible gene for HGF. However, no mutation in SOS1 gene was found in this family.