This paper will describe chiefly the pathology of the various types of chronic cystic mastitis as exposed by an exploratory incision. The more detailed clinical and other studies are reserved for a second paper. CLASSIFICATION OF CHRONIC CYSTIC MASTITIS For the present, I have subdivided this lesion of the breast into the following types, giving my abbreviations and nomenclature for each type: BB-12-1,-2,-3: Single or multiple cysts in chronic cystic mastitis, 210 cases. BB-13-4,-5,-6,-7,-8: Chronic cystic mastitis without large cysts, 140 cases. BB-12-1: The blue-domed cyst, 174 cases. BB-12-2: The cyst of the galactocele type, 8 cases. BB-12-3: The multiple blue-domed cysts in one or both breasts (diffuse cystic disease of the breast), 28 cases. BB-13-4: The nonencapsulated adenomatous area, 48 cases. BB-13-5: The nonencapsulated area of chronic cystic mastitis, containing one or more minute cysts, or one or more dilated ducts, or both, 39 cases. BB-13-6: The diffuse dilatation