A 78-year-old man presented with marked thrombocytosis (126.4 x 10(4)/microliters), low neutrophil alkaline phosphatase (NAP) score and an abnormal karyotype of 46, XY, del(20) (q11q13) (18 of 20 cells), without obvious anemia or ringed sideroblasts in bone marrow. He received ranimustine (MCNU) with a diagnosis of essential thrombocythemia. After 2 years, he was admitted because of macrocytic anemia. The peripheral blood smear showed anisopoikilocytosis with a few nucleated red blood cells. Moderate thrombocytosis (71.7 x 10(4)/microliters) and a low NAP score were also observed. Bone marrow aspiration revealed erythroid hyperplasia with a significant increase in ringed sideroblasts (85% of erythroblasts). Cytogenetic studies showed the same abnormal karyotype 46, XY, del(20) (q11q13) in 100% of metaphase cells as those at initial diagnosis. A diagnosis of sideroblastic anemia preceded by essential thrombocythemia was made. No rearrangement or amplification of c-src was revealed. The observation of the same chromosome abnormality (20q-) in different phases of this patient's disease indicates that chronic myeloproliferative disorders and myelodysplastic syndrome may share some borderline or transitional cases with a similar pathogenesis.