Some hematological neoplasms share features of myelodysplastic syndromes (MDS) and myeloproliferative disorders (MPD), the World Health Organization (WHO) classification of hematopoietic neoplasms introduced a new separate category: myelodysplastic/myeloproliferative diseases (MDS/MPD). Some patients may present diagnostic features of refractory anaemia with ringed sideroblasts (RARS) together with a markedly elevated platelet count (>600x109/L). Provided that these patients do not have del(5q) nor abnormalities of chromosome 3 (3q21q26) and because features of both myelodysplastic and myeloproliferative processes are present, the WHO classification considers this a provisional entity belonging to the MDS/MPD, unclassifiable category. The so-called provisional entity acquired sideroblastic anaemia associated with thrombocytosis/refractory anaemia with ringed sideroblasts (figure 1) associated with marked thrombocytosis (RARS-T) represents a heterogeneous disease, not only in terms of clinical and morphologic features, but also in terms of cytogenetic and molecular findings.