Eight cases of primary non-Hodgkin's lymphoma of the breast found in the pathology files of the Institute of Oncology, Ljubljana, Yugoslavia, for a period of 25 years (from 1961 to 1985) were analyzed. During the same period 5711 cases of breast carcinoma were seen. Seven cases were diffuse lymphomas, and one case was nodular. Five cases were high-grade large cell lymphomas and three cases were of low-grade type. One case of the latter group was associated with elevated IgA in the serum and showed monoclonal reaction of plasmacytic lymphoma cells for kappa light chain and IgA. In this case amyloid deposits were seen in the breast tumor. In six cases, focal infiltration of ductal/lobular epithelium by lymphoma cells was found. This so-called lymphoepithelial lesion appears to be an important characteristic of various mucosa-associated lymphoid tissue lymphomas which have been described in different extranodal sites (e.g. gastrointestinal tract, and respiratory tract). They may show different behavior from nodal counterparts, especially in terms of their spread to other mucosal sites which may appear before or without nodal dissemination. The breast appears to be yet another location for these lymphomas.