Mammary duct dilation is a common chronic breast disease with a long course, complex and diverse lesions. It is common in middle-aged and elderly women, with a peak age of 50-60 years old. It was once named "venous dilation", "plasma cell mastitis", "acne mastitis", etc. It was named "mammary duct dilation" because Haagensen and Stout believed that plasma cell infiltration was only an inflammatory reaction in the late stage of the disease based on its pathological characteristics. Its initial lesions and pathological characteristics are mammary duct dilation as its basic lesion. Recently, some scholars believe that plasma cell mastitis is not an inevitable process of mammary duct dilation. Plasma cell mastitis has its characteristic morphology and clinical manifestations, and it is regarded as a special type of mastitis.