Treatment will start between 5-6 months after your stem cell transplant. If you are assigned to the rituximab group, you will receive a total of 4 cycles of treatment. Each cycle will be given 6 months apart. Each cycle is made up of 4 doses of rituximab given once a week for 4 weeks. If the disease returns or intolerable side effects occur while on treatment, you will be taken off the study. You may still be able to receive rituximab outside of this study if your treating physician decided that it is okay.