For children undergoing bone marrow transplantation, respiratory failure carries mortality expectations well above 60%. The researchers have published preliminary evidence that continuous hemofiltration may greatly improve survival, if filtration is begun when the child first fulfills clinical criteria for ARDS. This is a departure from standard practice, as hemofiltration is usually begun later in the course (if at all) when multiple organ failure is entrenched. Hemofiltration, a `renal replacement therapy` for critically ill patients, is a slow, continuous process in which a semi-permeable membrane removes plasma water and solutes (up to about 35 kiloDaltons). Many cytokine and chemokine molecules are smaller than the molecular weight limit of the filter; hemofiltration might remove a critical amount, attenuating the unregulated inflammatory response responsible for respiratory failure and progression to multiple organ failure and death. The researchers will conduct a multi-center randomized trial assessing the effect of hemofiltration on survival from respiratory after bone marrow (or more precisely, hematopoietic stem cell) transplantation. The researchers will perform sensitive proteomic assays of serum and ultrafiltrate, to detect the presence of cytokines and chemokines known to be active in idiopathic pneumonia syndrome. Resulting profiles will constitute a uniquely complex description of ultrafiltrate and may provide evidence for modulation of immune function by hemofiltration.