The goal of this clinical research study is to find out if giving (Rituxan) rituximab with 90Y (ibritumomab tiuxetan) (90 Y Zevalin®) may be effective in treating low-grade lymphoma. The safety of this combination treatment will also be studied.

90 Y Zevalin and rituximab are both designed to attach to lymphoma cells, causing them to die.

Before you can start treatment on this study, you will have "screening tests." These tests will help the doctor decide if you are eligible to take part in this study. You will have a physical exam. Blood (about 2 to 3 teaspoons) and urine will be collected for routine tests. You will have a chest x-ray and computerized tomography (CT) scans of the neck, chest, abdomen (stomach area), and pelvis.

A PET scan is also recommended. A PET (Positron Emission Tomography) scan is a medical technique that monitors the activity in the brain and other organs by tracking the movement of a special radioactive solution through the body. The radioactive solution is either inhaled as a mist or injected into a vein. The radioactive solution is usually made from simple sugar that has radioactive particles attached to it. After the solution is injected into a vein or inhaled, the PET scanner takes pictures of the radioactive solution as it moves through the body and collects in various organs. By watching how the solution travels through the body and studying where the solution collects, researchers can learn the extent of disease in certain organs in the body.

You will have an electrocardiogram (ECG -- a test that measures the electrical activity of the heart). You will have a bone marrow aspirate and biopsy performed. To collect a bone marrow aspirate and biopsy, an area of the hip or chest bone is numbed with anesthetic, and a small amount of bone marrow and bone is withdrawn through a large needle. Women who are able to have children must have a negative blood or urine pregnancy test.

If you are found to be eligible to take part in this study, you will be given diphenhydramine (Benadryl) by vein and you will take acetaminophen (Tylenol) by mouth before each dose of rituximab. This is done to help decrease the risk of developing side effects of rituximab. You will then receive 1 dose of rituximab by vein over about 4-6 hours on Day 1 of treatment. After treatment with rituximab, you will then be given 111 In Zevalin (this is a radioactive agent that binds to rituximab to help with imaging exams), by vein over about 10 minutes. This is so researchers can use a special camera to see where the drug is in your body.

You will have imaging performed (on a camera, like an x-ray) on either Day 2 or 3. On Day 8 (7 days after the first dose of rituximab) you will receive a second dose of rituximab. You will also be given Benadryl, Tylenol, and 90Y Zevalin in the same manner as on Day 1.

If you experience intolerable side effects while on this study, you may be removed from this study. Your treatment on the study will end on Day 8.

You will return for follow-up tests for 4 years. Blood (about 2 tablespoons) will be drawn weekly for the first 3 months. Blood (about 2 tablespoons each time) will also be drawn at month 6 and 9 of the first year, and every 6 months in the second, third, and fourth years. You may also have CT scans, PET scans (which are recommended), x-rays, and bone marrow biopsies and aspirates performed, if your doctor thinks they are necessary.

Your participation on this study will end in about 4 years.

This is an investigational study. 90 Y Zevalin and rituximab are FDA approved and commercially available. Their use in this study is investigational. Up to 36 patients will take part in this multicenter study. Up to 36 patients will be enrolled at M. D. Anderson.

Inclusion Criteria:

1. New diagnosis of low-grade indolent lymphomas Stage I-II. Patients with multiple skin lesions will be eligible provided that the skin is the only site of involvement.
2. Histology includes Indolent cluster of differentiation antigen 20 (CD20)+ lymphomas including: Follicular lymphoma, Extranodal marginal lymphoma of MALT type, Nodal Marginal zone B-cell lymphoma (+/- monocytoid cells), and Splenic marginal B-cell lymphoma (+/- villous lymphocytes).
3. Signed Informed Consent.
4. Age >/= 18 years.
5. Pre-study Zubrod performance status of 0, 1, or 2.
6. Acceptable hematologic status within two weeks prior to patient registration, including: absolute neutrophil count ([segmented neutrophils + bands] x total WBC) >/= 1, 500/mm^3, total lymphocyte count </= 5,000/mm^3 and platelet counts >/= 100,000/mm^3.
7. Female patients who are not pregnant or lactating.
8. Men and women of reproductive potential who are following accepted birth control methods (as determined by the treating physician, however abstinence is not an acceptable method).
9. Patients determined to have < 25% bone marrow involvement with lymphoma within six weeks of registration (define measurement of a bone marrow aspirate or biopsy).
10. Patient should have at least one lesion measuring >/= 1.5 cm in a single dimension. Measurable cutaneous lesions are allowed.

Exclusion Criteria:

1. Presence of central nervous system (CNS) lymphoma.
2. Patients with HIV or AIDS-related lymphoma.
3. Patients with pleural effusion.
4. Patients with abnormal liver function: total bilirubin > 2.0 mg/dL.
5. Patients with abnormal renal function: serum creatinine > 2.0 mg/dL.
6. Patients who have received prior external beam radiation therapy to > 25% of active bone marrow (involved field or regional).
7. Impaired bone marrow reserve as indicated by < 15% bone marrow cellularity
8. Serious nonmalignant disease or infection which, in the opinion of the investigator and/or the sponsor, would compromise other protocol objectives.
9. Major surgery, other than diagnostic surgery, within four weeks.
10. Evidence of transformation in the latest biopsy.

Adverse events collected during treatment period including time from first Rituxan infusion (through Day 8) up to 12 weeks following Zevalin infusion.