Background: Standard of care immunochemotherapy in front-line (1L) follicular lymphoma (FL) is highly efficacious but not without significant toxicity. High rates of grade 3-5 adverse events (AEs), primarily infection and bone marrow suppression, are experienced in up to 75% of patients. A more tolerable but equally effective approach is required. PD-1 inhibition, in combination with rituximab (R), increases T cell anti-tumour effect & enhances NK cell antibody dependent cell cytotoxicity, with proven efficacy in relapsed FL. The concept of ‘priming’ the immune system with nivolumab (N) prior to tumour-directed therapy has rationale and evidence, but the safety of this approach in 1L FL is not described. Methods:‘1^st FLOR’ (NCT03245021) is an open-label, multi-centre, phase 2, Simon’s 2-stage study of N + R (N = 39). Key eligibility were stage III-IV grade 1-3A FL requiring 1L systemic therapy; ECOG ≤2; adequate organ function. All patients (pts) receive induction N 240mg IV 2-weekly for 4 cycles. Pts with complete response (CR) receive 4 further cycles of 240mg IV N monotherapy then 12 cycles of maintenance N 480mg IV 4-weekly. Pts with < CR had 240mg N plus 375mg/m² IV R 2-weekly for 4 cycles followed by maintenance N+R (N 480mg 4 weekly for 12 cycles; R 12 weekly for 8 cycles). Primary endpoint (EP) was ≥ G3 toxicity rate during induction. Secondary EPs; response rate by Lugano response criteria, overall toxicity, PFS, OS. Results: Between September 2017 to March 2020, 39 pts were enrolled. Baseline characteristics included median age of 54 (range: 28-79). stage IV disease in 67%, B Symptoms & bulk (≥7cm) in 23% each, intermediate-high risk FLIPI in 74%. The primary EP was met, with only 16 pts (41%) having ≥G3 toxicity at end of induction. Non-immune AEs were predominantly G1-2; most commonly infection (67%) & fatigue (64%). G3-4 Immune-related AEs were infrequent and included pancreatitis plus hepatitis (N = 1), pancreatitis alone (N = 1), rash (N = 1), transaminitis (N = 2), hypocortisolism (N = 1), hyperglycaemia (N = 3) and asymptomatic lipase/amylase increase (N = 3). Median follow-up was 17.5 months (range: 7-39). Overall response rate was 92% (36/39) with CR in 54% (21/39). Median time to CR was 5 months (m) (range: 2-25). Nine pts (23%) discontinued treatment; 7 due to progressive disease (1 pt died of transformed FL), 2 developed constitutional symptoms (1 stable disease, 1 partial response). In 25 evaluable pts, 12m PFS & OS is 72% (CI 51-88) & 96% (CI 80-100). Biomarker analysis is in progress. Conclusions: Immune-priming with single-agent N, then combination N+R in 1L FL is associated with favourable toxicity and high ORR & CR rates potentially providing an alternative to chemotherapy. Acknowledgements: Bristol-myers Squibb provided funding and nivolumab for this study. Clinical trial information: NCT03245021