Granulomatous mastitis (GM) is a chronic inflammatory disease of the breast with granulomas as the main pathological feature, which was first reported by Kessler in 1972. Due to the rarity of the disease and the lack of adequate knowledge about it, misdiagnosis and mismanagement often occur in clinical practice. In order to improve the diagnosis and treatment of GM, this article introduces its etiology, pathogenesis and clinicopathological characteristics. 1. Etiology and pathogenesis So far, the etiology and pathogenesis of GM are still unclear, but there have been more studies on the factors related to its development. It is generally believed that GM is an autoimmune disease and is related to the use of contraceptives by patients; Kiefer et al. reported that it is related to rod-shaped bacillus infection; Brown et al. believed that it is an immune reaction and local hypersensitivity reaction caused by breast milk; FletcHER et al. believed that the disease is related to hormonal imbalance in the body such as hyperprolactinemia or inflammation of lobular granuloma caused by infection and chemical stimulation; in addition, GM is also related to mycobacteria and Although there is no uniform understanding of the specific pathogenesis of GM, it can be found that abnormal hormones such as estrogen contained in contraceptives can stimulate the development of breast ducts, progesterone can stimulate the development of lobular alveoli, and lactogen can promote the secretion of milk, and high levels of lactogen can cause milk secretion to the alveoli, but due to the lack of posterior lobe prolactin, it is not possible to secrete milk to the alveoli. The lack of posterior lobe prolactin prevents milk from draining from the lobules into the ducts, thus causing milk to stagnate in the lobules, causing the products of lipid-like material decomposition to occur in the lobules locally as a result of hypersensitivity and immune reactions, resulting in lobular granulomatous inflammation. The average age of GM patients is about 32 years. The lesion usually occurs in the unilateral breast and can occur in all parts of the breast except the areola area, but it is more common in the upper quadrant. The lump is the main manifestation of GM and is characterized by solitary, sudden and rapid enlargement; the lump is hard, with a length of 1.5-10 cm, and in huge cases, the whole breast may be involved; the lump has unclear borders, the surface is not smooth, and it may adhere to the skin or surrounding tissues; the local skin may appear red and swollen, but it is not obvious and the skin temperature is not high. Most GMs have breast pain, which is often vague and rarely severe. Systemic symptoms are often not obvious, and a few may be accompanied by low fever, fatigue, sleep disorder, loss of appetite, joint pain and skin erythema. If left untreated, GM may develop abscesses within a short period of time (about 2 weeks). Abscesses can be large or small, single or multiple, with multiple microabscesses common; multiple abscesses are usually associated with subtle sinus tracts. If the abscess is allowed to develop, after about 1 month, it may cause skin ulceration and pus overflow, which will form sinus tracts and remain untreated for a long time, resulting in serious disfigurement of breast appearance. 3, pathological features GM histological examination can be seen in the cut surface diffuse distribution of corn to soy-sized dark red nodules, some nodules can be seen in the center of the small cystic cavity. Microscopically, the lesions are seen to be centered in the lobules of the breast, with a multifocal distribution, and most of the terminal ducts or alveoli of the lobules disappearing, with common neutrophilic foci, i.e. microabscesses. Occasionally, small focal necrosis was seen, but no caseous necrosis was present. Antacid staining does not reveal Mycobacterium tuberculosis, and there are no obvious foam cells or dilated ducts. Cytologic examination reveals more neutrophils, lymphocytes, Langan giant cells or foreign body giant cells, nuclear debris, and epithelial cells. Although GM is rare, the incidence of GM is increasing year by year with the aggravation of environmental pollution, changes in living and eating habits, increased mental stress, lactation disorders, and abuse of sex hormones. Due to the diverse clinical manifestations and lack of specificity of GM, it is difficult to diagnose and treat, so it should attract sufficient attention. Ultrasound and coarse needle aspiration histological examination can help to make a clear diagnosis.