Osteoma, a benign neoplasia of bone tissue, is considered a quiescent neoplasm. An active osteoma is an uncom- mon lesion. We presented a case of a mandibular osteoma with an unusual active behavior. In January of 2001 was admitted in the Oral Medicine Service a 11-years old female because an asymptomatic growth in the left side of their face with two years of evolution. Radiographical examination revealed a well circumscribed spherical radio- paque lesion of approximately 2 x 2 cm in diameter on lateral side of the left angle of the mandible. Microscopically the surgical sample coming from incisional biopsy showed normal bone tissue. A diagnosis of osteoid osteoma was emitted. Surgical treatment was indicated and refused by the parents of the patient. In August 2007 the same was readmitted because an increase in their facial asymmetry. Radiographical examination revealed an increase in the size of the radiopaque area to 3x3 cm. A 99m Tc MDP bone scintigraphy was indicated and performed. Because the scintigraphy showed bone activity the surgical elimination of the whole lesion was indicated and done. Mi- croscopically the lesion was composed by vital mature lamellar and trabecular bone. A diagnosis of giant active osteoma of the mandible was emitted. The maxillofacial surgeon should awake by the possibility of an osteoma shows neoplastic activity.