With increasing emphasis on theearly diagnosis of cancer it is obvious that the pathologist will observe increasingly early histologic manifestations of this disease. In recentyears it has become apparent to us that various forms of carcinoma in situ were being encountered in ever greater frequency and in locations where such phenomena were hitherto but rarely discovered. Examples of entirely noninfiltrative lesions of a definitely cancerous cytology have been accumulated for almost every mucosalined structure.

Carcinoma in situ in the breast is a disease which has been recognized for many years. The term, however, has not been employed and for the usual form of the disease the designation" noninfiltrative comedo-carcinoma" has served. Nevertheless, comedo-carcinoma constitutes an example par excellence of carci-noma in situ of a glandular organ. This form, however, is a disease mainly of the larger duct system. One is much less apt to think of carcinoma in situ as a disease of small lobular ducts and lobules. The latter process is relatively rare. One of us (F. WS) had occasion several months ago to conduct a clinicalpathological symposium t on tumors of the breast at which a lesion of this type was presented. It was found that the malig-nant character of the process was not recognized by a number of pathologists. For this reason it is felt desirable to review certain features of such tumors. An impression of the incidence of this type may be gained through a survey of the mammary cancers observed during the past year at the Memorial Hospital. There were two typical examples of strict carcinoma in situ of lobules and terminal lobular ducts in approximately 300 primarv, operable, mammary cancers. Additional examples of the lesion have been seen during