The purpose of this study is to investigate the effectiveness of intravenous fosaprepitant therapy to reduce nausea and vomiting during the treatment of high dose interleukin-2 (HD IL-2) therapy for metastatic melanoma or metastatic renal cell carcinoma. Fosaprepitant is an intravenous (IV) medication that is FDA- approved for use in adults for the prevention of nausea and vomiting during chemotherapy. Fosaprepitant works by blocking the neurokinin-1 receptor, which is a receptor in the brain that is known to cause nausea and vomiting. Past studies estimate that up to 70% of patients undergoing treatment with HD IL-2 will have nausea and/or vomiting. While fosaprepitant has been used in clinical practice to treat nausea and vomiting during HD IL-2, there have not been any studies done to see how well it works. All patients will receive treatment (IV fosaprepitant) during the study during either the first or second hospital admission for HD IL-2. On the admission that the subject is not receiving IV fosaprepitant, the subject will receive placebo (a medicine that looks like fosaprepitant, but is not active). The study is double-blinded, which means neither the subject, nor the study doctor will know to which group you have been assigned to that admission (IV fosaprepitant or placebo). This study design was chosen to limit the potential for bias, which means the trial was designed to try to ensure that unknown factors do not affect trial results. When patients start the study, patients will be randomly assigned to one of two groups: those who receive treatment (IV fosaprepitant) first and those who receive placebo first. During the first admission, subjects will be given the IV fosaprepitant or IV placebo during admission. During the second admission, subjects will 'crossover' and receive the other treatment that they did not receive during the first admission. Improvement in nausea and vomiting will be assessed by counting the number of nausea and vomiting episodes, recording if the subject needs additional medication for nausea and vomiting, and by using patient questionnaires.