We surveyed IL-21 receptor (IL-21R) in leukemia and lymphoma and found that follicular lymphoma cells showed exceptionally high IL-21R expression. Notably, IL-21 showed divergent effects depending on the cell origin: growth stimulation in Burkitt lymphoma cell lines and adult T cell leukemia/lymphoma cell lines but induction of apoptosis in B lymphoma cell lines with t(14;18)(q32;q21), a marker karyotype of follicular lymphoma. IL-21 activated caspase-8 and -3 and reduced mitochondrial membrane potential. More importantly, IL-21 decreased Bcl-2 expression but increased Bax expression. These results support a new therapeutic approach using the IL-21/IL-21R system in follicular lymphoma.