T HE interest in the case described herein centers about the association of a lymphocytic leukemoid reaction with carcinoma of the breast. This combination of lesions presented a baffling clinical picture not clarified until the postmortem examination disclosed diffuse metastatic involvement of marrow, lymph nodes, spleen and liver from the primary carcinoma in the breast. The simulation of the leukemic state or the clinical syndrome of leukemia by an unrelated clinicopathologic condition is spoken of as a leukemoid state or reaction. It should be stressed that uncommon as the leukemoid reactions of the myeloid type are in carcinoma lymphocytic leukemoid reactions are a far greater rarity. Krumbhaar3 and others2 do not discuss malignant tumors in the production of lymphocytic leukemoid reactions while referring to measles, pertussis, acute infections with lymphocytosis and such chronic lesions as tuberculosis as producing the lymphocytic leukemoid state. A study of the American literature and a partial survey of the foreign literature discloses only six specific references to lymphocytic leukemoid reactions associated with malignant or benign neoplasms.7-10*15*16 Winan’s caselo is the only one referring to a benign tumor in the production of a lymphocytic leukemoid reaction. However, it is difficult to consider his case as definitely a lymphocytic leukemoid reaction since the resemblance to infectious mononucleosis leaves doubt as to the nature of the lymphocytosis. Lisa, Solomon and Gordon7 reported a case of carcinoma of the lung with diffuse metastases to the bone marrow, spleen and liver that was clinically and hematologically indistinguishable from lymphatic leukemia. The white cell count was 28,450 with 60 per cent lymphocytes, chiefly prolymphocytes. Reich’s case* of adenocarcinoma of the sigmoid showed extensive replacement of the bone marrow and generalized visceral metastases. No specific reference to splenic metastasis is made. Clinically, the white blood count ranged from 18,700 to 53,800, lymphocytes 77 to 95 per cent. An antemortem bone marrow biopsy was described as showing marked infiltration with lymphocytes, many of which were abnormal. Autopsy disclosed diffuse involvement of the marrow by metastatic carcinoma but no histologic evidence of leukemia in any organs. Sala and Stein9 presented a case of ductal carcinoma of the breast which at autopsy showed most of the marrow replaced by tumor and diffuse involvement of the spleen and liver. The white blood cell count ranged from 29,600 to 33,400, 60 to 65 per cent lymphocytes, young and mature, and some abnormal granulocytes. This case showed active extramedullary hematopoiesis in the liver and spleen. Muller and Werthemann15 report a case of carcinoma of the breast with extensive metastases to the bone marrow, spleen and lymph nodes which during life presented a white blood cell count of 33,000 with 63 per cent lymphocytes. Recently there has appeared in the * From the Mallory Institute of Pathology, Boston City Hospital, Boston, Mass.