A 33-year-old woman was hospitalized because of bleeding tendency. Hemoglobin was 10.7 g/dl, white blood cell 2,100/microliters and platelet 2.1 X 10(4)/microliters. Bone marrow showed marked dysplasia of trilineage blood cells. Atypical blasts and monocytoid cells accounted for 14.5% in the myelogram. Cytogenetic study of bone marrow cells revealed translocation with t(11;21) in all of 20 metaphasic cells analyzed by G-banding method. A diagnosis of RAEB was made. Familial survey revealed that her elder brother died of acute monocytic leukemia (AMoL). The patient received small dose therapy of Ara-C and BHAC-DMP therapy, but a remission was not obtained. The patient's general condition deteriorated with infection, bleeding tendency and chronic hepatitis due to transfusions, therefore we have followed up the patient with prednisolone and red blood cell transfusion. It has become evident that some types of acute leukemia with monocytic features have a cytogenetic change at 11 q 23. But it is rare that RAEB with increased monocytoid cells has a cytogenetic change at 11q23. In addition, the patient's elder brother died of AMoL. This case is important in relation to cytogenetic change at 11q23 and hematopoietic abnormalities.