A hard lump under the ear is a symptom of many diseases. Common diseases include sebaceous cysts and swollen lymph nodes behind the ear. 1. Sebaceous cyst: This disease is mostly caused by the obstruction of sebaceous gland ducts, which leads to the accumulation of contents in the glands and the formation of round swellings that protrude towards the skin surface. If secondary infection occurs, the patient is often accompanied by pain, redness and swelling. 2. Enlarged lymph nodes behind the ear: this disease is mostly caused by infection, inflammation, tumor and other reasons, resulting in enlarged lymph nodes behind the ear, which are often accompanied by obvious swelling and tenderness. It is recommended to go to the hospital in time to complete the relevant examinations to clarify the cause of the disease and then give the appropriate treatment program.