Feline myelodysplastic syndromes (MDS) has been diagnosed in many cats infected with feline leukemia virus, although the pathogenesis of this hematopoietic deficiency has been unclear. In this study, we assayed the bone marrow erythroid colony-forming units (CFU-E) and granulocyte-machrophage CFUs (CFU-GM) to investigate the pathogenesis of feline MDS. The number of CFU-E colonies was decreased in 4 of 7 cats with MDS, and the number of CFU-GM colonies was also decreased in 4 cats. Furthermore, small colonies of CFU-GM were found in all 7 cases. These findings indicated that refractory cytopenia of feline MDS could be caused by abnormal maturation and differentiation of hematopoietic stem cells in bone marrow, as it is in human MDS. The pathogenesis of feline MDS might be similar to that of human MDS.