There is a hard lump behind the left ear. Clinically, this condition is mainly seen in the following diseases: 1. Inflammation of the lymph nodes behind the ear. This condition may originate from the irritation of the ear, throat, parotid gland, and other parts of the body, causing the lymph nodes to swell, in which case there is mostly a painful sensation. When ultrasound is done, the lymph nodes can be found to be enlarged. In this case, if there is no pain, it can be observed. If there is painful irritation, anti-inflammatory drugs can be applied for symptomatic treatment, and after the inflammation subsides, the lymph nodes will gradually disappear; 2. sebaceous cysts behind the ear, which may be more frequent in patients who often eat fatty foods, often surf the Internet, and stay up late, can be found inside the cyst-like changes when doing ultrasound, and sometimes can be seen inside with changes of cystic fluid. In this case, small cysts can be observed, while large cysts can be treated surgically. The possibility of other diseases, such as lipomas, for example, is not excluded.