A 77-year-old-man was diagnosed with follicular lymphoma (FL), grade 3A. After six courses of R-THP-COP (rituximab, pirarubicin, cyclophosphamide, vincristine and prednisolone) therapy, he achieved complete remission (CR). He achieved a second CR after radiotherapy, a third CR after six courses of bendamustine /rituximab (BR) therapy, and a fourth CR after six courses of BR therapy. However 2 months after the last chemotherapy, his tumor progressed rapidly and he died. Autopsy results showed medium and large lymphoid cells with pleomorphic, irregular nuclei and prominent nucleoli infiltrated in multiple lymph nodes, the liver, the lung, and the spleen. The lymphoid cells were positive for CD3, CD8, granzymeB, TIA-1 and negative for CD4, CD20, CD79a, CD10, and CD56. Autopsy diagnosis was peripheral T-cell lymphoma, not otherwise specified. Occurrence of lymphoma in T-cell lineage should be considered, if the course of low-grade B-cell lymphomas, such as FL rapidly progresses.