A bump on the back of the head can have many possible causes, including injuries, cysts, fatty growths, inflamed hair follicles, and bone spurs. Some bumps on the back of the head may be hard or soft, and they can vary in size. The human skull has a natural bump on the back of the head called an "inion," which marks the bottom of the skull where it attaches to the neck muscle. Some common causes of a bump on the back of the head include:

• Injury: Falling backward, impacts or collisions during contact sports, violence, hitting the head against the headrest of a car seat in a traffic accident, and other types of accidents can cause bumps on the back of the head. A blow to the back of the head can cause a scalp hematoma, which is where a collection of blood just beneath the skin forms a semisolid bump.

• Cysts: Epidermoid cysts and pilar cysts are two types of cysts that can cause bumps on the back of the head. Epidermoid cysts are small, hard lumps that develop beneath the skin, while pilar cysts are usually found on the scalp and are filled with keratin.

• Folliculitis: Inflammation of the hair follicles can cause bumps on the back of the head. Folliculitis can be caused by an infection of the follicles by fungi, bacteria, viruses, or inflammation from ingrown hairs.

• Bone spurs: Bone spurs are bony projections that can develop on the bones of the spine and can cause bumps on the back of the head.

• Lipoma: Lipomas are fatty tissue growths under the skin that are usually soft or rubbery to the touch. They are fairly common and can occur on the back of the head.

Most bumps on the back of the head are harmless, but in rare cases, a bump on the head could indicate a more serious problem. If you notice changes with the bump on your head, if its bleeding or is painful, contact your doctor. To diagnose the cause of the bump or lump on your head, a doctor might conduct a physical exam, blood test, skin biopsy, or imaging, such as an X-ray or CT scan. If necessary, your clinician will investigate it further through medical imaging tests or biopsies.