A bulge on the left side of the back of the neck may be a sebaceous cyst, lipoma, myofasciitis, skin infections, and so on. 1. Sebaceous cysts: they are cysts formed by sebaceous gland duct obstruction and sebum accumulation, and usually grow slowly without conscious symptoms. 2. Lipoma: it is a swelling formed by benign proliferation of adipose tissue. Lipoma is soft and slow-growing, without pain and other discomforts, and the surface skin color is normal. 3. Myofasciitis: If there is a history of long-term neck strain, accompanied by neck stiffness and pain, it is considered to be a localized nodule caused by myofasciitis of the neck. 4. Skin infections: If accompanied by local redness, swelling, heat and pain, it is likely to be folliculitis, boils, carbuncles and other purulent skin infections. A bulge on the left side of the back of the neck may have other causes, only by the symptoms of a bulge on the left side of the back of the neck can not be diagnosed, it is recommended to go to a regular hospital interview, under the guidance of a professional physician examination and treatment.