RATE-36 INTRODUCTION Primary central nervous system lymphoma (PCNSL) is a subtype of non-Hodgkin’s lymphoma that involves the brain, spinal cord or leptomeninges, without evidence of systemic disease. This rare disease accounts for ~3% of all primary central nervous system (CNS) tumors. Methotrexate-based regimens are the standard of care for this disease with overall survival rates ranging from 14 to 55 months. Relapse after apparent complete remission can occur. We sought to understand the outcomes of patients who relapsed. METHODS This is an IRB-approved investigation of patients treated at our institution between 12/31/2004 and 10/12/2016. All cases of PCNSL were retrospectively identified as part of a database registry and data were evaluated for demographic information, absence or presence of relapse, location of relapse, treatment regimens, and median time to relapse. RESULTS Forty-four patients were identified with a diagnosis of PCNSL. Mean age at diagnosis was 63.1 yrs (range 20-86, SD=13.2 yrs). Of the 44 patients, 28 patients successfully completed an initial treatment regimen without recurrence or toxicity requiring a change in therapy. Relapse occurred in 11 patients with the location of relapse being in the CNS only (n=5), eye only (n=1), outside CNS only (n=3), or a combination of CNS and outside CNS (n=2). Sites of relapse outside of the CNS included testes (n=1), lung (n=1), adrenal gland (n=1), kidney/adrenal gland (n=1), and retroperitoneum (n=1). Median time to relapse from successful completion of therapy (95% CI) was 12.9 months (95% CI: 4.8-48.6 months). CONCLUSION After successful initial treatment, PCNSL can relapse and this relapse can occur in the CNS and outside the CNS. Vigilant monitoring of off-treatment patients with a history of PCNSL is necessary to guide early diagnosis of relapse and to initiate aggressive treatment.