A lump behind the ear in a child may be the result of gill slit cysts, sebaceous cysts, lymphadenitis, and other conditions. 1. Gill slit cyst: usually due to incomplete degeneration of the gill slits during fetal life. The first gill slit cyst often occurs in Pochet’s triangle, up to the bottom wall of the external auditory canal, and the second gill slit cyst often occurs in the plane below the back of the mandibular angle, so some patients may show a lump behind the ear. 2. Sebaceous cysts: usually due to the follicle sebaceous gland mouth blockage, and sebaceous glands are distributed throughout the body, often in the head, face, back buttocks and other parts of the body, if the sebum behind the ear collection, can also appear cysts. Its shape is round, medium hardness or elasticity, small like a grain of rice, large like a peanut or egg, slow growth. 3. Lymphadenitis: It is usually due to bacterial infection and is also related to cold and upper respiratory tract infection. Patients often have symptoms of swollen and painful lymph nodes, and there are also lymph nodes behind the ears, if lymphadenitis occurs, there may be a lump behind the ear. If the patient’s condition is serious, an abscess can also form. It is recommended that parents should pay attention to the condition of children with a lump behind the ear, and find out the cause of the disease and then treat it actively.