A small invisible lump behind the ear may be caused by enlarged lymph nodes behind the ear, sebaceous cyst behind the ear, lipoma behind the ear and so on. 1. Enlarged lymph nodes behind the ear: it may be caused by inflammation, and the common ones are enlarged lymph nodes behind the ear caused by acute otitis externa, acute otitis media, mumps, as well as tuberculous enlarged lymph nodes or metastatic cancer of lymph nodes. 2. Posterior sebaceous cysts: sebaceous cysts formed when the sebaceous gland ducts behind the ear are blocked and the discharge of secretions is obstructed. 3. Posterior auricular lipoma: the tumor has a very thin envelope, soft texture, and irregular shape, such as round, lobulated or bag-shaped. If the tumor is large, liquefaction, solid change or focal hemorrhage can occur, and a large number of mature fat cells can be seen microscopically, and fibrous tissue and mucus can be seen, which are often called fibro-lipoma and muco-lipoma. There is a small lump behind the ear which cannot be seen.