 Lumbar punctures LP are performed in approximately one thousand oncology patients per year at the Hospital for Sick Children. In a previous study, we determined the optimal dose of remifentanil which provides effective anesthesia with little or no movement during LP in children. The present study will determine the optimal dose combination of propofol and remifentanil to keep patients comfortable and still during the procedure while decreasing the incidence of side effects and allows for shorter recovery times and earlier discharge from the recovery room. 