Hematopoietic neoplasms involving the breast, although less common than breast carcinoma, are often clinically indistinguishable from other breast tumors. Microscopically, these tumors can mimic primary carcinoma of the breast, especially in limited material such as needle biopsy specimens. Forty-five hematopoietic breast neoplasms including 21 primary non-Hodgkin's lymphomas (NHLs), 19 secondary NHLs, 1 undetermined NHL, 1 lesion secondary to Hodgkin's disease, and 3 granulocytic sarcomas were reviewed with regard to histologic subtype, morphologic features, and immunophenotype. The median age of patients at presentation was 67 years for those with primary NHLs and 61 years for those with secondary NHLs. The majority of lymphomas were intermediate or high grade. Diffuse large cell type was by far the most common histologic subtype. No lymphomas resembling lymphomas of mucosa-associated lymphoid tissue were identified in this series, suggesting that such lymphomas are rare in breast compared with other sites such as the gastrointestinal tract and lung. Four primary NHLs, 2 secondary NHLs, and 1 granulocytic sarcoma were initially misdiagnosed as carcinomas; three patients underwent radical mastectomy, and at least three other patients nearly received surgical treatment. One primary anaplastic large cell lymphoma of B-cell origin was identified that closely resembled poorly differentiated ductal carcinoma. "Single file" or targetoid patterns with extensive sclerosis mimicking invasive lobular carcinoma was common in lymphomas and was seen in one of the granulocytic sarcomas. In addition, two breast lymphomas, one of B-cell phenotype and the other of T-cell phenotype, showed frequent signet ring cells, which potentially could be confused with lobular carcinoma. Despite a number of recent articles on lymphomas of the breast, it appears that these tumors continue to be confused with carcinomas. Histologic features suggestive of lymphomatous involvement include absence of in situ carcinoma, frequent individual karyorrhectic cells, lymphoepithelial lesions, and cellular discohesiveness.