Inclusion Criteria:

• Understand and voluntarily sign an informed consent form
• Able to adhere to the study visit schedule and other protocol requirements
• Histologic diagnosis of CD20+ B-cell NHL including diffuse large B-cell lymphoma, follicular lymphoma, mantle cell lymphoma and other B-cell lymphomas excluding chronic lymphocytic leukemia
• Received high-dose chemotherapy with autologous stem cell transplantation (ASCT) from 42 to 128 days before enrollment with stable disease, partial response or complete response following ASCT
• All previous cancer therapy, including radiation, hormonal therapy and surgery, must have been discontinued at least 4 weeks prior to treatment in this study
• ECOG performance status of =< 2 at study entry; Karnofsky performance status of >= 70% at study entry
• Absolute neutrophil count >= 1,500/mm^3
• Platelet count >= 75,000/mm^3
• Serum creatinine =< 2.0 mg/dL
• Phase I subjects must have estimated or measured creatinine clearance >= 60 ml/min
• Phase II subjects must have estimated or measured creatinine clearance >= 30 ml/min
• Total bilirubin =< 1.5 mg/dL
• AST (SGOT) and ALT (SGPT) =< 2 x ULN or =< 5 x ULN if hepatic metastases are present
• Disease free of prior malignancies for >= 5 years with exception of currently treated basal cell, squamous cell carcinoma of the skin, or carcinoma "insitu" of the cervix or breast
• Females of childbearing potential (FCBP) must have a negative serum or urine pregnancy test with a sensitivity of at least 50 mIU/mL within 10 - 14 days prior to and again within 24 hours of prescribing lenalidomide for cycle 1 (prescriptions must be filled within 7 days)
• Must also either commit to continued abstinence from heterosexual intercourse or begin TWO acceptable methods of birth control, one highly effective method and one additional effective method AT THE SAME TIME, at least 28 days before she starts taking lenalidomide
• FCBP must also agree to ongoing pregnancy testing
• Men must agree to use a latex condom during sexual contact with a FCBP even if they have had a successful vasectomy
• Able to take aspirin (81 or 325 mg) daily as prophylactic anticoagulation (subjects intolerant to ASA may use warfarin or low molecular weight heparin)
• All study participants must be registered into the mandatory RevAssist program, and be willing and able to comply with the requirements of RevAssist

Exclusion Criteria:

• Any serious medical condition, laboratory abnormality, or psychiatric illness that in the opinion of the investigator would prevent the subject from providing written informed consent
• Pregnant or breast feeding females (lactating females must agree not to breast feed while taking lenalidomide)
• Use of any other experimental drug or therapy within 28 days of baseline
• Known hypersensitivity to thalidomide
• The development of erythema nodosum if characterized by a desquamating rash while taking thalidomide or similar drugs
• Known hypersensitivity to rituximab
• Concurrent use of other anti-cancer agents or treatments
• Known positive for HIV or infectious hepatitis, type B or C
• Residual grade 3 or grade 4 non-hematologic toxicity after ASCT
• Transfusion requirement (red blood cells or platelets) within 14 days prior to baseline
• Use of hematopoietic growth factor (including filgrastim, pegfilgrastim, sargramostim, erythropoietin, or darbepoetin) within 14 days prior to baseline
• Any other condition not defined above, including the presence of laboratory abnormalities, which in the opinion of the investigator would place the subject at unacceptable risk if he/she were to participate in the study, or would confound the ability to interpret data from the study
• Prior use of lenalidomide either concurrently with rituximab or within 8 weeks following a dose of rituximab
• Concomitant use of other anti-cancer therapies, including radiation, thalidomide, or other investigational agents is not permitted while subjects are receiving protocol therapy during the treatment phase of the study
• Corticosteroid therapy also is not permitted while subjects are receiving protocol therapy during the treatment phase of the study