This will be a single center, comparative, randomized, double-blind, phase III trial designed to evaluate the efficacy of the NK-1 antagonist, aprepitant (MK-869), in combination with ondansetron and dexamethasone in the prevention of acute and delayed nausea and vomiting compared to ondansetron and dexamethasone in patients receiving highly emetogenic preparative regimens prior to autologous or allogeneic (related and unrelated) stem cell transplantation.