• Biopsy-confirmed (fresh or archival tissue) follicular lymphoma grade 1-3A that is CD20+ (by immunophenotype or immunohistochemistry) at time of diagnosis. All degrees of CD20 positivity will be accepted. Composite high-grade lymphoma will be excluded.

• Subjects must have measurable disease at time of enrollment as defined by at least one lymph node with long axis ≥1.5 cm and short axis \>1.0 cm and Deauville ≥ 4 seen on baseline PET/CT

• Stage III/IV at initial diagnosis

• 1 prior line (at least 3 cycles) of systemic upfront or first-line therapy consisting of anti-CD20 antibody (e.g. obinutuzumab or rituximab) combined with chemotherapy (e.g. bendamustine, CHOP, CVP, or lenalidomide). Rituximab monotherapy, rituximab plus radiation, or radiation alone is not sufficient.

• Subjects need to have achieved a partial response or stable disease as best response following upfront treatment. Subjects with progressive disease will be excluded.

• Subjects must have completed all prior anti-lymphoma therapy at least 4 weeks (28 days) prior to start of epcoritamab.

• Age ≥18 years.

• ECOG performance status ≤ 2

• Life expectancy of greater than 2 years

• Participants must meet the following organ and marrow function as defined below:

‣ Absolute neutrophil count ≥1000 cells/mcl (G-CSF allowed if marrow involved with disease)

⁃ Platelets ≥75,000 cells/mcl (transfusion allowed if marrow involved)

⁃ Hemoglobin ≥ 8 g/dL (transfusion allowed if marrow involved)

⁃ Total bilirubin ≤ 1.5 institutional upper limit of normal (ULN). In patients with suspected/known Gilbert's disease total bilirubin up to 3x ULN will be allowed

⁃ AST(SGOT)/ALT(SGPT) ≤3× institutional ULN unless suspected/known involvement by follicular lymphoma

⁃ Creatinine ≤ institutional ULN OR creatinine clearance \> 45 ml/min (by Cockcroft-Gault estimate or 24-hr creatinine clearance measurement)

• Patients with hepatitis B core antibody positivity with negative PCR on antiviral therapy will be eligible but will be required to receive appropriate antiviral prophylaxis as described in Section 5.4. Patients with Hepatitis C antibody must have undetectable viral load.

• Participants with a history of prior malignancy will be eligible if all treatment of that malignancy was completed at least 2 years prior to enrollment to this study, the treatment was considered curable-intent, and there is currently no evidence of disease.

• Resolution of toxicities from prior therapy to baseline or grade ≤1 (with the exception of grade 2 peripheral neuropathy or any grade alopecia)

• Ability to understand and the willingness to sign a written informed consent document.

• Females of childbearing potential must agree to practice a highly effective method of birth control (as defined by the EU Clinical Trial Facilitation Group) consistent with local regulations regarding the use of birth control methods for patients participating in clinical trials:

‣ Established use of oral, injected or implanted combined (estradiol and progesterone containing) hormonal contraception;

⁃ Placement of an intrauterine device (IUD) or intrauterine system (IUS);

⁃ Male partner sterilization (the vasectomized partner should be the sole partner for that patient)

⁃ True abstinence (when this is in line with the preferred and usual lifestyle of the patient)

• Women must agree not to donate eggs (ova, oocytes) for the purposes of assisted reproduction during the trial and for 12 months after receiving the last dose of epcoritamab. Men must also not donate sperm during the trial and for 12 months after receiving the last dose of epcoritamab.

• A man who is sexually active with a woman of childbearing potential must agree to use a barrier method of birth control (i.e. use of condom) during the trial and for 12 months after receiving the last dose of epcoritamab.