A hard bump on the back of the neck is commonly caused by the following reasons: first, sebaceous cyst formation, the disease is caused by hair follicle obstruction, often due to repeated local friction, sweating more, etc., generally need to go to the hospital to perform ultrasound for clarification, the disease requires surgery to be able to be eradicated, otherwise it will lead to local redness and swelling and ulceration, and may flow pus. Second, osteophytes, because the back of the neck is the cervical spine, osteophytes can be manifested as a local hard mass, which can be performed cervical X-ray, CT, etc. to clarify. Third, the local insect bite caused by. Fourth, local lymph node enlargement, when the lymph node enlargement, local hard swelling can be touched, sometimes accompanied by pain, local skin redness. Fifth, there are other rare causes, such as allergies or autoimmune system diseases. In short, if the abnormality persists, you need to go to the hospital promptly to clarify the cause.