What is the most sensible way of organizing the disorderly spectrum of acquired marrow failure states collectively known as the myelodysplastic syndromes (MDS)? While the 2008 version of the World Health Organization classification is the current standard, the recent proliferation of MDS prognostic tools illustrates the usefulness of supplemental information for clinical purposes. Many cases of acquired bone marrow failure do not fit cleanly into established MDS categories, yet an alternative diagnosis is not apparent. The term "idiopathic cytopenias of undetermined significance" (ICUS) has been proposed to describe these cases, but there is a paucity of information about the natural history of ICUS. New data on the natural history of MDS associated with a broad range of cytogenetic abnormalities that were not included in the International Prognostic Scoring System (IPSS), as well as the emerging picture of karyotypically occult DNA changes, promise to inform future classifications.