True dentigerous cysts are rare one originated from the epithelial cells left in the periodontal membrane, and contain whole or part of a permanent tooth and occasionally more than one tooth, the crown of which projects into the cavity. Recently author has experienced a case of infected dentigerous cyst with a inverted tooth. The severe infection of this cyst was arised from trauma on the face and made difficult to diagnose this disease to be a dentigerous cyst. We had treated this case successfully by the Caldwell Luc' procedure.