A hard lump behind the ear in infants may be caused by diseases such as lymph nodes or dermoid cysts. 1. If an infant develops upper respiratory tract infection, inflammation of the external ear canal, tympanitis, otitis media, mumps and other infectious diseases, it may cause inflammation, enlargement and hardening of lymph nodes draining from the head and neck area such as behind the ear, in front of the ear and parotid gland. 2. In infants with dermatoid cysts behind the ear, there are subcutaneous or dermal nodules, which are hemispherical, hard, and large in size, and usually have no conscious symptoms. Infants with unexplained hard lumps behind the ear are recommended to go to the hospital specialist in time, after the doctor to determine the nature and cause of the disease, targeted treatment, do not use drugs privately.