A bulge behind the ear in children may be clinically associated with the following diseases: First, lymphadenitis behind the ear, which is the most common clinically, may be caused by inflammatory infections in the adjacent areas, such as otitis media or mumps, that involve the lymph nodes, resulting in enlargement of the lymph nodes, thus causing this manifestation. Secondly, some patients with congenital cyst infection or cysts formed by congenital embryonic residue may also have this manifestation, which is not very common clinically. Thirdly, some patients with localized osteomatous hyperplasia of the mastoid process can also appear bulging and show a very hard feeling. Fourthly, some patients have purulent otitis media, which is not treated in time, and the disease is aggravated by the formation of extracranial complications, such as subperiosteal abscesses of the mastoid process, which can manifest bulging. The diagnosis is not difficult to confirm by CT of the mastoid process of the middle ear, which is a localized fluctuating sensation during clinical examination. In conclusion, a bulge behind the ear in children should be clinically determined by combining the medical history, physical examination and relevant ancillary tests.