Chemotherapy-induced nausea and vomiting (CINV) are among the most bothersome symptoms during cancer treatment according to children and their parents. Most children receiving hematopoietic stem cell transplant (HSCT) conditioning experience CINV despite receiving antiemetic prophylaxis. Olanzapine improves CINV control in adult cancer patients, has a track record of safe use in children with psychiatric illness, does not interact with chemotherapy and is inexpensive. We hypothesize that the addition of olanzapine to standard antiemetics will improve chemotherapy-induced vomiting (CIV) control in children receiving high dose cyclophosphamide for hematopoietic stem cell transplantation (HSCT) conditioning.