We report two patients who had been initially diagnosed as having a myelodysplastic syndrome but subsequently progressed into a leukothrombocytosis state which mimicked a chronic myeloproliferative disorder. Both patients had anemia and mild neutropenia without thrombocytopenia at the time of their diagnosis of myelodysplastic syndrome, and dyshematopoietic features were present in the bone marrow. After treatment with vitamin D3 for 7 and 18 months, respectively, they developed leukothrombocytosis which responded to hydroxyurea. We speculate that these and other similar patients with this unusual course might constitute an entity distinct from the typical myelodysplastic syndromes or chronic myeloproliferative disorders.