A round hard thing behind the ear may be caused by enlarged lymph nodes, sebaceous cyst, subcutaneous lipoma and other diseases. 1. Swollen lymph nodes: swollen lymph nodes behind the ear are considered to be related to inflammatory infections, such as otitis media, gingivitis, rubella, ocular herpes zoster, lymph node tuberculosis may lead to swollen lymph nodes behind the ear, and a round hard lump may appear. 2. Sebaceous cysts: The cause of sebaceous cysts is relatively clear, which may be caused by the blockage of sebaceous gland ducts, the secretion can not be discharged normally in the local collection, and can occur behind the ear. Small cysts are usually asymptomatic, but they gradually increase in size and may rise above the surface of the skin, resulting in the appearance of localized round and hard bumps, 3. Subcutaneous lipoma: Subcutaneous lipoma is a benign tumor of the skin, which is composed of mature adipose tissue, and if it occurs behind the ear, it may lead to the appearance of round hard things locally. If it occurs behind the ear, it may lead to the appearance of round and hard things behind the ear, and it may also be normal mastoid tissue. It is recommended not to squeeze and scratch, and you can go to the hospital to understand the cause and make a clear diagnosis.