A hard lump on the back of the neck may be caused by boils, epidermal cysts, acne and other diseases. 1. Boil: It is mostly caused by Staphylococcus aureus infection, or may be induced by excessive sweating, scratching, poor hygiene habits and other factors. They may appear locally as small red nodules with painful symptoms and can be found on the head, face and neck. 2. Epidermoid cyst: it is a kind of cyst containing keratin in the dermis, and the etiology is related to trauma, surgical operation and other factors. Cysts of different sizes can appear in the neck, back, buttocks and other parts of the body, the texture is hard, when the secondary infection can be localized redness, swelling and pain. 3. Acne: mainly related to the change of androgen level, sebaceous gland secretion increase and other factors, the patient may appear pimples, cysts, nodules and other symptoms, can appear in the head, chest, back and neck and other places. There are many causes of a hard lump on the back of the neck, and patients are advised to identify the cause.