Phase 3 Study of Zevalin Following R-CVP in Previously Untreated Patients With Follicular Non Hodgkin's Lymphoma (NHL)

Inclusion Criteria: * Must give written informed consent and any authorizations required by local law (e.g., Protected Health Information). * Age greater than or equal to 18 years at the time of informed consent. * Histologically confirmed follicular NHL according to the Revised European American Lymphoma (REAL)/World Health Organization (WHO)classification (from initial diagnosis); grades 1, 2, or 3. * Bi-dimensionally measurable lesion(s) in at least one site. * High risk NHL as defined by a follicular lymphoma international prognostic index (FLIPI) of 3, 4, or 5 assessed within 3 months prior to randomization. * NHL requires treatment as determined by the investigator. * Confirmed CD20+ lymphoma cells. * Eastern Cooperative Oncology Group (ECOG) performance status of 0, 1,or 2. * Expected survival of greater than or equal to 3 months. * Male subjects and female subjects of child bearing potential willing to practice effective contraception during the study and willing and able to continue contraception for 1 year after their last dose of study treatment (R CVP for subjects in the observation arm and the Zevalin therapeutic regimen for subjects in the Zevalin arm). Exclusion Criteria: * Previous anticancer treatment for NHL, including chemotherapy, immunotherapy, radiation (locoregional or extended field), radioimmunotherapy, or investigational therapy. * Known seropositivity for hepatitis C virus, hepatitis B virus (surface antigen-positive), or other active infection uncontrolled by treatment. * Known diagnosis of human immunodeficiency virus infection. * Presence of primary gastric, central nervous system (CNS), or testicular lymphoma, or transformed lymphoma, or chronic lymphocytic leukemia (CLL). * Active therapy within previous 5 years for other malignancy, except non-melanoma skin tumors or stage 0 (in situ) cervical carcinoma. * Abnormal liver function: total bilirubin \>1.5 X upper limit of normal (ULN) or ALT \>2.5 X ULN. * Impairment of renal function (serum creatinine \>1.5 X ULN) not due to lymphoma. * Concurrent severe and/or uncontrolled medical disease (e.g., uncontrolled diabetes, congestive heart failure, myocardial infarction within 6 months of study, unstable and uncontrolled hypertension, chronic renal disease, or active uncontrolled infection) that could compromise participation in the study. * Known hypersensitivity to murine and/or chimeric proteins. * History of severe allergic or anaphylactic reactions. * Known allergy to any components present in rituximab, cyclophosphamide, vincristine, and prednisone (CVP), or Zevalin. * Treatment with another study treatment or approved therapy for investigational use within the 12 weeks prior to randomization. * Exposure to monoclonal antibodies, cytokines, growth factors, soluble receptors, other recombinant products, or fusion proteins. * Females with a positive pregnancy test result at screening or who are currently breastfeeding. * Inability to comply with study requirements. * Major surgery within 28 days except for diagnosis. * In need of immediate intervention to treat life threatening complications.