The relationship between pulpal and periodontal disease was first described by Simring and Goldberg in 1964. Pathological agents pass between the pulp and periodontium, thereby creating the endoperio lesion. Lateral canals play an important role in the spread of infection from pulp to periodontium. This article reports a case of a 38-year-old patient with a combined endoperio lesion. Bone density is a key factor in predicting bone healing. Application of Hounsfield unit to determine the healing pattern of bone. How to cite this article Hegde MN, Hegde ND, Shrivastava D. Application of Hounsfield Unit to evaluate Prognosis of a Rehabilitated Tooth with Grade Three Mobility. World J Dent 2012;3(1):109-111.