The main causes of a lump on the left back side of the neck are considered to be related to posterior cervical lymphadenitis, lipoma, folliculitis or molluscum contagiosum, etc. The treatment plan needs to be chosen according to the cause of the disease. 1. Posterior cervical lymphadenitis: If there is infection in the head, face and neck, and the infected foci drain to the back left side of the neck through the lymphatic fluid, the lymph nodes will be enlarged, often accompanied by local pain, and it is usually easy to produce localized lumps. 2. Lipoma: benign hyperplasia of subcutaneous fat in the back of the left side of the neck, forming a local subcutaneous mass, which is easy to form the symptom of a lump on the back of the left side of the neck. 3. Folliculitis: if the back of the neck left part of the follicular inflammation caused by bacteria or fungi, often manifested in the hair follicle as the center of the papules, pustules, and so on, thus forming the phenomenon of the back of the neck there is a pimple. 4. molluscum contagiosum: molluscum contagiosum, for the fibrous tissue of the superfluous organisms, often in the back of the neck of the left side of the part of the skin surface of the high tibial proliferation, so as to form the back of the neck there is a pimple of the performance. It is recommended to go to the dermatology department of the regular hospital in time to get a clear diagnosis, under the guidance of the doctor to standardize the treatment, usually pay attention to keep the skin hygiene and cleanliness, pay attention to eat less spicy and too much greasy food during the treatment period.