There are many reasons why there is a hard lump behind one ear, such as enlarged lymph nodes, sebaceous cysts, tumors and so on. 1. Enlarged lymph nodes: Inflammation of parotid gland, tonsils, throat, middle ear and outer ear can cause enlarged lymph nodes, so there will be a hard lump behind one ear. 2. Sebaceous cysts: there are sebaceous glands behind the ear, if the sebaceous gland secretion causes local blockage, it can cause sebaceous cysts, which will show the phenomenon of a hard lump behind one ear. 3. Tumor: a hard lump behind one ear may belong to tumor, such as trichoplasmoma, fibroma, lipoma and so on. There are many reasons why there is a hard lump behind one side of the ear, and it is recommended to go to the hospital in time to improve the examination and take standardized treatment according to the doctor’s instruction after the symptom appears, so do not ignore the symptom and take it lightly.