Myelodysplasia must be considered in the differential diagnosis of patients who have bone marrow failure, but bone marrow cellularity per se may not substantially affect either response to therapy or prognosis. It is unclear whether the primary pathophysiologic defect differs between hyper- and hypoplastic patients who have myelodysplasia. Cellularity does not seem to affect response to immunosuppressive therapy significantly and does not seem to be the major factor affecting improvements in response to lenalidomide, stem cell transplantation, or hematopoietic growth factors.