Course 1 (odd courses) will start by giving rituximab by vein over 6 hours on Day 1 and Day 11 for patients whose leukemia or lymphoma expresses CD20. Participants will receive the drugs acetaminophen (Tylenol) and diphenhydramine hydrochloride (Benadryl) 30-60 minutes before each dose of rituximab. This will be done to lessen the risk of fever, chills, and allergic reactions. Usually, the first dose of rituximab requires about 6 to 8 hours to complete. If side effects do occur during the infusion, participants will be observed for an additional 2 hours after the rituximab is given. Then, the cyclophosphamide will be given as described below.