Eight years ago my associates and I operated in a case of what was believed at the time to be mammary carcinoma. There was present a hard, diffuse mammary tumor. The nipple was retracted. The breast had the peau d'orange appearance typical of carcinoma. The axillary nodes were much enlarged and quite hard. There was no question in the minds of the several clinicians who examined this patient as to the diagnosis of carcinoma, as nearly all of the classic diagnostic signs were present. A radical amputation was performed. Examination of the specimen later in the laboratory, to our complete surprise and chagrin, revealed no cancer. Dr. Ewing, after making a careful study, rendered the diagnosis of "plasma cell mastitis," a term entirely new to us. He named this lesion plasma cell mastitis on account of the overwhelming preponderance of the plasma cell. This cell is not prominent in the