A small hard lump can be felt on the left side of the back of the neck for a variety of reasons, and the common causes include lymphadenitis, sebaceous cysts, soft tissue tumors in the neck, lymph node metastasis from other parts of the tumor, and tumors in the lymph nodes themselves. 1. Lymphadenitis: generally caused by bacterial infection or viral infection, leading to inflammatory reaction of lymph nodes, resulting in small hard lumps on the left side of the back of the neck, which may be accompanied by slight pain. 2. Sebaceous cysts: blockage of the sebaceous gland ducts in the body and accumulation of glandular contents lead to the formation of sebaceous cysts, which can be manifested as small hard lumps on the left side of the back of the neck. Usually accompanied by pain, local redness and other symptoms. 3. Neck soft tissue tumor: neck soft tissue tumors such as lipoma, fibroma, leiomyosarcoma, nerve sheath tumor, etc., can be manifested as small hard lumps on the left side of the back of the neck. If they are benign tumors, they usually grow slowly and without pain. 4. Tumor lymph node metastasis and lymph node tumor itself: tumors in other parts of the body, such as breast cancer, metastasized to the lymph nodes in the neck, as well as lymph node tumors themselves, such as lymphoma, can be manifested as a small hard lump on the left side of the back of the neck. If you can feel a small hard lump on the left side of the back of the neck, it can also be seen for other reasons. It is recommended to go to the hospital in time, combined with your own symptoms and laboratory imaging tests, to clarify the cause of the disease and diagnosis, and timely treatment.