Syndrome of abnormal chromatin clumping in leucocytes syndrome (ACCLS) is an uncommon entity which shares clinical and biological features with the myelodysplastic (MDS) and chronic myeloproliferative syndrome. In fact, as some authors consider ACCLS a new type of MDS, others maintain that it is in Ph'negative/bcr-abl negative chronic myeloid leukaemia. A new case of ACCLS appeared in a 68 year old woman, who presented with anaemic symptoms, bleeding and recurrent infections, and a haematological picture including progressive macrocytic anemia, thrombocytopenia and leuco-erythroblastosis. Marrow hypercellularity with granulocytic hyperplasia, and mature granulocytes presenting nuclear hyposegmentation and large peripheral blocks of chromatin separated by clear zones were the characteristic features of this case. No cytogenetic abnormalities were found and DNA flow-cytometry content was normal (euploid), supporting the thought that a disequilibrium exists in the hetero-chromatin/eucromatin ration in AACLS. Reverse PCR for bcr-abl transcripts was negative. The cell-cycle-phase analysis showed a high fraction of S-cells in the bone marrow (27%) in contrast to a very low S-phase (0.2%) in the peripheral blood, pattern that is different from both CMML and CML. In vitro clonogenic assays showed a high colony forming capacity and a certain grade of autonomous proliferation of the bone marrow cells, which is reminiscent of the CMML growth behaviour in culture. The patient was treated with vitamin D3, low dose Ara-C, prednisone and hydroxyurea until her demise, fifteen months after diagnosis. In total, the patient received 47 units of packed cells and 114 of platelet concentrates, and was transfused only when she presented anaemic or hemorrhagic symptoms. These clinical and haematological features suggest that ACCLS is a distinct entity that should be considered a sixth type of MDS, beside CMML, with which it has much in common.