Eight patients with primary carcinoid tumor of the breast are reported. One patient had two primary tumors in the same breast and another had bilateral carcinoids. All patients were females (mean age 47.5) presenting with a painless mass in the breast. The carcinoid syndrome was not present in any case. The tumors were grossly firm and well circumscribed. Histologically, they were composed of small cells arranged in solid nests separated by fibrous tissue, resembling carcinoids of other sites. Argyrophil granules were noted in each case. Electron microscopic study done in three cases showed small, uniform, membrane-bound “neurosecretory” granules measuring 250 nm in average diameter. The treatment in all patients was radical mastectomy. Three of four patients with tumors measuring 3 cm or more died with widespread metastases (1, 2.5, and 16 years after diagnosis). One is alive with disseminated disease 14 years later. Four patients with tumors measuring 1–2 cm treated more recently are alive with no evidence of disease 6 months to 3.5 years after surgery. The finding of intraductal carcinoma in three of our cases plus the identification by others of cells containing silver deposits in normal ducts of the breast, would suggest this anatomical structure as the possible site of origin of these tumors. Carcinoids arising in the breast must be distinguished from infiltrating duct, medullary, and lobular carcinomas in addition to carcinoids metastatic to this site.