Refractory anemia with ring sideroblasts associated with marked thrombocytosis (RARS-T) is a provisional entity in the current World Health Organization classification and is thought to be a myelodysplastic/myeloproliferative neoplasm (MDS/MPN). We analyzed 18 cases of RARS-T. All patients had thrombocytosis (platelet count, 515-1,100 × 10(3)/μL [515-1,100 × 10(9)/L]) and anemia (hemoglobin level, 7.2-12.6 g/dL [72-126 g/L]). Three patients had mild leukocytosis (WBC count, 3,900-16,300/μL [3.9-16.3 × 10(9)/L]). Ring sideroblasts were 8% to 75% in the bone marrow. Megakaryocytes showed a spectrum of morphologic findings. JAK2(V617F) was identified in 9 of 15 cases, including 7 of 9 with thrombocytosis (platelet count, >600 × 10(3)/μL [600 × 10(9)/L]) and 1 with 8% ring sideroblasts. The MPL(W515L) mutation was not detected (n = 9). We conclude that RARS-T is a pathogenetically heterogeneous group of limited diagnostic usefulness. Approximately 60% of cases carry JAK2(V617F)and seem to be closer to an MPN in which ring sideroblasts may be a secondary phenomenon. The remaining cases usually lack the JAK2(V617F)mutation, have a platelet count less than 600 × 10(3)/μL (600 × 10(9)/L), and may represent an MDS or MPN with thrombocytosis of unknown mechanisms.