We report a rare case of follicular lymphoma which rapidly showed transformation to the Burkitt type of lymphoma after a treatment consisting of chemotherapy and irradiation. A 51-year-old male visited our hospital in August 2000 because of bilateral neck lymphadenopathy. He was diagnosed as having follicular lymphoma (grade 2) (clinical stage IIIA) with complex karyotypic abnormalities involving t(14 ; 18)(q32 ; q21) and CD20 expression. Initially he was followed as an outpatient without chemotherapy. Six months later, he was admitted because of hydronephrosis due to an intrapelvic tumor. He underwent chemotherapy with 4 courses of CHOP regimen following irradiation therapy and a partial response was obtained. Four months after initiation of the treatment, his disease recurred with numb chin syndrome. Bone marrow aspiration revealed bone marrow involvement by lymphoma cells which had a Burkitt-like appearance. A cytogenetic study using bone marrow blood showed complex abnormalities involving t(8 ; 22)(q24 ; q11) in addition to t(14 ; 18). In spite of salvage chemotherapy, the patient died in September 2001.