Based on the in vitro experiments which showed the capacity of Aracytine (AraC) to induce differentiation of leukemic cells, low dose AraC treatment of patients with acute myeloid leukaemia (AML) or myelodysplastic syndromes (MDS) has been available since 1980. Many studies involving only small numbers of patients have demonstrated contradictory results regarding the efficacy or the mechanism of action of this regimen. A retrospective clinical study is presented, including 159 patients treated in 11 French institutions and 1 German institution (AML: 99 patients; MDS: 38 patients). The study confirms the efficacy of low dose AraC, especially in elderly patients with hypoplastic AML. Although, less severe than with high dose chemotherapy, toxicity needs regular monitoring. The mechanism of action cannot be ascertained by this clinical study, however half of the complete remissions were obtained with no bone marrow aplasia. Studies with low dose AraC and/or other in vitro differentiating agents need to be continued.