Lymphocytic mastitis and diabetic mastopathy are uncommon fibroinflammatory breast diseases. The lesions seen in these entities are unique in that the associated lymphoid infiltrates are composed of predominantly B cells. In addition, B-cell lymphoepithelial lesions, a finding commonly associated with extranodal marginal zone B-cell/mucosa-associated lymphoid tissue (MALT) lymphomas, are also often present in lymphocytic mastitis and diabetic mastopathy. Although the clinical and immunomorphologic features are well characterized, the clonality of the B-cell infiltrate and the lymphomatous potential of lymphocytic mastitis and diabetic mastopathy have not been emphasized in the literature. We evaluated 11 cases of lymphocytic mastitis/diabetic mastopathy for immunoglobulin heavy chain gene rearrangement and correlated the findings with all available clinical data. A longstanding history of Type I diabetes mellitus was present in seven patients. One nondiabetic patient had Sjogren's syndrome, and two patients had no history of diabetes mellitus or other autoimmune disease. Clinical data were unavailable for one patient. B-cell-predominant lymphoid infiltrates were seen in all cases, and B-cell lymphoepithelial lesions were found in five. No evidence of a B-cell clone was found in any of the 11 cases by appropriately controlled immunoglobulin heavy chain gene rearrangement studies, and none of the patients developed lymphoma during follow-up intervals ranging from 2-126 months. These findings suggest that despite the presence of B-cell-predominant lymphoid infiltrates and lymphoepithelial lesions, lymphocytic mastitis and diabetic mastopathy do not appear to be associated with an increased risk for lymphoma.