Histologic transformation of low-grade B-cell lymphoma to diffuse large B-cell lymphoma is associated with poor prognosis. Although plasma cell differentiation is common in these lymphomas, an overt plasmablastic transformation (PBL-T) has been only rarely reported. We report 6 cases of PBL-T occurring in 3 chronic lymphocytic leukemias (CLL) and 3 follicular lymphomas. Five patients were men, and the mean age was 65 years (range, 52 to 72 y). None of them had history of immunodeficiency. In 3 cases the PBL-T occurred 34 to 85 months after the initial diagnosis, and in 3 it was detected simultaneously with the small cell component at diagnosis. All patients received chemotherapy after transformation, and 4 died 4 to 24 months after this diagnosis. In 3 cases, PBL-T occurred in an extranodal site. All PBL-Ts had immunoblastic morphology with admixed plasma cells, were CD20 and PAX5 negative, expressed λ light chain, and 5 were CD138 positive. All cases were negative for HHV8, and only 1 PBL-T was Epstein-Barr virus positive. Evidence of a clonal relationship between the small cell and PBL-T components was found in 5 cases. In 2 CLL cases, both components had 13q deletions, and in all follicular lymphoma cases both components harbored the t(14;18) translocation. MYC translocations were observed in 2 cases transformed from a CLL. In conclusion, PBL-T expands the clinicopathologic spectrum of the transformation of low-grade B-cell lymphomas. These transformed tumors are clinically, histologically, and phenotypically similar to primary plasmablastic lymphomas, but they are not associated with immunodeficiency and rarely have Epstein-Barr virus infection or MYC alterations.