The myelodysplastic syndromes are a heterogeneous group of hematopoietic stem cell diseases in which both diagnosis and prognosis are determined by cellular morphologic criteria. In some patients, prognosis is poor due to proliferation of immature cells, i.e. development of acute leukemia. An important clinical decision for patients with myelodysplastic syndromes is whether to treat with supportive care or to use cytoreductive drugs to control the proliferative component of these illnesses. Two cases of chronic myelomonocytic leukemia are presented where cell surface antigen phenotype analysis showed characteristics suggestive of proliferative disease and the patients were treated and obtained remission with cytoablative therapy. Cell surface marker analysis may be useful in guiding therapeutic decisions in myelodysplasia.