Dentigerous cyst is the most common developmental odontogenic cyst which is associated with an impacted tooth and are commonly diagnosed at an early age. If unnoticed they can grow and expand into a large lesion. In this article, we report a case of a large dentigerous cyst which was aggressive and expansile involving the mandible, crossing the midline and associated with a partially developed, supernumerary tooth. The patient was a 48-year-old male, who had noticed a swelling in the oral cavity for 2 years. The patient was treated with cystic enucleation and the postoperative healing was satisfactory.