We reported a case of chronic B-cell leukemia reacted to the administration of 1 alpha (OH)D3 (Alfarol-CHUGAI Pharm. Co.), The patient showed pancytopenia with IgM-kappa type monoclonal protein in the serum. The bone marrow aspiration was failed due to a dry tap, but the biopsied specimen showed a marked infiltration of small sized lymphoid cells with wide cytoplasm. The leukemic cells from peripheral blood showed a morphology of atypical hairy cells, Surface markers of the leukemic cells were IgM, D(kappa)+, CD 19+, CD 20+, CD 21- and HLADR+, The leukemic cells showed no L-tartrate resistant acid phosphatase sensitivity. This case was diagnosed as a chronic B-cell leukemia closely related to a hairy cell leukemia. The treatment with estrogen-chlorambucil compound (Bestrabucil-KUREHA Chem, Co.) or splenic irradiation was not effective. However, after two months' administration of Alfarol the regular blood transfusion was not needed because of increment of the Hb concentration. After eight months of its administration, the bone marrow aspirate showed a marked decrease in the number of the leukemic cells and a restoration of normal hematopoietic cells. This experience suggested that Alfarol in usefull for the treatment of chronic B cell leukemia including hairy cell leukemia and chronic lymphocytic leukemia.