The hypothesis of the study is that propofol-fentanyl sedation for lumbar punctures in children with acute leukemia/lymphoma results in fewer adverse events than propofol sedation alone. Secondary hypotheses state that propofol-fentanyl sedation results in a better sedation induction, recovery profile and is preferred by patients/families. The study is a double blind, randomized, placebo controlled crossover study. Following the induction phase of chemotherapy, children will be randomized in a crossover manner to receive either fentanyl-propofol or propofol-placebo for future lumbar punctures. Patients will be studied on two separate occasions, once with propofol-placebo (normal saline) and once with propofol-fentanyl. Children will be monitored continuously by pulse oximetry, electrocardiogram (ECG) and direct nursing and physician observation during the sedation in accordance with the University of Wisconsin (UW) Pediatric Policy and Procedure. Propofol will be titrated to a Children's Hospital of Eastern Ontario Pain Scale (CHEOPS) score of ≤ 7 for all children. Oxygen saturation, respiratory rate, heart rate and blood pressure will be recorded every 3 minutes by a study investigator during sedation.