Blastic natural killer (NK) cell lymphoma corresponding to CD4+CD56+ malignancies is a novel disease entity, according to the results of clinical, morphologic, and immunologic studies. It is especially noteworthy that this disease likely arises from plasmacytoid dendritic cells (pDCs), described previously as plasmacytoid T-cells, which have an important role in innate and adaptive immunity. However, the exact relationship between the tumor cells and pDCs remains to be elucidated.We encountered a patient with typical blastic NK cell lymphoma, which later converted to leukemic manifestations, and tried to establish a cell line using the leukemic cells.We succeeded in establishment of a novel cell line,CAL-1, which originated from the primary malignant cells.The genetic and phenotypic features of CAL-1 cells bear a similarity to those of pDCs, namely, plasmacytoid morphology at light and electron microscopy; negative results for CD11c and lineage-associated markers of CD3, CD14, CD19, and CD16; positive results for HLA-DR, CD4, CD56, CD45RA, and CD123; and negative results for TCR and IgH gene rearrangements. An interesting finding was that CAL-1 cells change morphologically into the mature DC appearance with many long dendrites after short-term culture in the presence of granulocyte-macrophage colony-stimulating factor and interleukin 3.CAL-1 cells can secrete tumor necrosis factorαbut not interferon α.Thus although they do not share in part phenotypic and functional features with their normal counterparts, CAL-1 cells mostly exhibit a striking pDC phenotype.We describe the first novel pDC cell line of CAL-1.This cell line should open the opportunity for study not only of CD4+CD56+ tumor cells but also of pDCs in vitro.