Inclusion Criteria:

• Patients with previously untreated CD20+ CLL/SLL must have either Rai stage III/IV disease or be Rai stage I/II with evidence of disease activity as defined by the NCI 1996 guidelines
• Patient must be age 18 or older.
• Patient must have a ECOG performance status of 0, 1 or 2
• Patient must have an anticipated (untreated) survival of at least 3 months.
• Patient must have adequate:
• Bone Marrow Function: ANC ≥ 1,000/µL, platelets ≥ 75,000/µL.
• Renal Function: serum creatinine <2 times upper limit of normal.
• Hepatic function: total bilirubin <2 times upper limit of normal, AST <5 times upper limit of normal.
• Males and females must agree to use a contraceptive method from enrollment to 6 months after receiving I-131 labeled tositumomab.

Exclusion Criteria:

• Patients who have received prior radiolabeled antibody.
• Patients with active hemolysis.
• Patients must not require sustained transfusion support of blood products.
• Patients who have received cytotoxic chemotherapy, radiation therapy, immunotherapy, or cytokine treatment prior to study entry. Patients who have received systemic steroids within 1 week of study entry are excluded, except patients on maintenance steroid therapy for a non-cancerous disease.
• Patients who have undergone treatment with either stem cell or bone marrow transplant.
• Patients with active obstructive hydronephrosis.
• Patients with evidence of any significant systemic illness, active Hepatitis B infection or other active infection at the time of study entry.
• Patients with New York Heart Association class III or IV heart disease or other serious illness that would preclude evaluation.
• Patients with known HIV infection.
• Patients who are pregnant or nursing.
• Patients with prior malignancy other than CLL/SLL, except for adequately-treated skin cancer (basal cell or squamous cell carcinoma), in situ cervical cancer, or other cancer for which the patient has been disease-free for 5 years unless approved by the PI.
• Patients with known brain or leptomeningeal involvement by malignancy.
• Patients who have, in the opinion of the investigator, other medical, social, or psychosocial factors that may negatively impact compliance or their safety by participation in this study.