A small hard bag behind the baby’s ear may be related to diseases such as lymphadenitis behind the ear and epidermoid cyst. 1. Behind-the-ear lymphadenitis: When the body’s immunity is low, pathogenic microorganisms such as bacteria invade the body. The lymphatic system will produce resistance, which in turn leads to the enlargement of the lymph nodes behind the ear and the appearance of a small hard bag behind the ear. 2. Epidermoid cysts: Epidermoid cysts are cysts that occur within the dermis containing keratin. The disease may be related to skin damage that leads to the implantation of epidermal cells under the skin and other factors. There may be other reasons for the baby to have a small hard bag behind the ear, it is recommended to go to the hospital in time, improve the examination to clarify the cause of the disease, and then give targeted treatment or treatment.