Progression of follicular lymphoma (FL) to a higher-grade lymphoma occurs in 25% to 60% of cases and frequently indicates a poor prognosis. The transformation is accompanied by alteration in morphologic features and clinical behaviors. Herein, we report a case of FL occurring in a 59-year-old woman with the development of a precursor B-cell lymphoblastic lymphoma (PBC-LBL) during a 6-month period. Both lymphomas had identical bcl-2 and immunoglobulin heavy-chain gene breakpoints as assessed by polymerase chain reaction amplification and direct sequencing, indicating that the 2 tumors originated from a common precursor B-cell clone or the PBC-LBL arose from clonal evolution of its preceding FL and "dedifferentiated" into a lymphoblastic stage. Immunohistochemical studies revealed that both lymphomas displayed typical phenotypic characteristics. A c-myc gene translocation was observed in the PBC-LBL but not the FL by fluorescence in situ hybridization. The c-myc translocation is also reported in similar cases in the literature and likely is crucial in the pathogenesis of the PBC-LBL.