A bulge on the back of the neck may be a lipoma or a sebaceous cyst. A lipoma is a mass formed by the proliferation of fat under the skin, with fatty tissue inside and a membrane wrapping around it to form a mass outside. For small lipomas, they can be followed up and no surgery is needed for the time being. If the lipoma is large and there are symptoms of pressure or pain, surgery can be considered. A sebaceous cyst is a mass formed after the opening of the sebaceous gland has been blocked. The inside is a tofu-like substance, and the outer envelope is hard. Sebaceous cysts are prone to inflammation secondary to bacterial infection and even abscess formation, so surgery is recommended to remove them after a clear diagnosis.