A 73-year-old man, in clinical remission 17 years after radiation therapy for a localized low-grade follicular lymphoma (FL), developed extensive lymphadenopathy, ascites, and splenomegaly with splenic masses. Axillary lymph node biopsy showed FL composed of nodules of centrocytes side by side with nodules of immunoblasts rather than centroblasts. Immunophenotyping revealed conventional FL markers (BCL-2, BCL-6, and CD10) as well as MUM-1 in the immunoblastic component, suggesting postgerminal center differentiation. Fluorescence in situ hybridization showed t(14;18) in both centrocytic and immunoblastic components and a copy gain of BCL-6 predominantly in the immunoblastic component. Areas of centrocytic and of immunoblastic nodules were macrodissected separately and underwent molecular evaluation for immunoglobulin heavy chain gene rearrangement. Identical base-pair peaks were found, attesting to their clonal identity. This case represents a very unusual example of transformation of a low-grade FL to a nodular immunoblastic FL.