A subcutaneous breast cyst is a cystic mass located in the subcutaneous tissue of the breast, not a cyst located in a gland, so it is not due to accumulation of breast milk or hormonal changes in the body. A subcutaneous breast cyst can be a simple cyst or a cortical cyst. In the case of simple cysts, if they are small and asymptomatic, they can be followed up with regular ultrasound. If the cyst is large and palpable on examination, it can be surgically removed under local anesthesia, and the possibility of malignant lesions is relatively small. For sebaceous cysts, this is a retention cyst caused by obstruction of excretion of sebaceous glands, most commonly found in areas with dense distribution of sebaceous glands, and can also be seen in the mammary glands. On the surface, there are small black dots at the openings of the sebaceous glands, and inside the cysts, sebum and epidermal keratinized material are gathered to form a tofu scum. For subcutaneous sebaceous cysts of the mammary glands, surgical excision under local anesthesia is still recommended in this case before it becomes serious due to the susceptibility to infection. If infection occurs, sensitive antibacterial drugs should be applied, and topical Bactrim ointment can be used. If an abscess is formed, it should be incised and drained promptly, and in the case of a more limited early abscess, a separate shuttle incision can be made to remove it completely.