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e19545 Background: High-dose methotrexate (HD-MTX) at a dose between 2.5 to 5 gm/m2 is commonly administered in conjunction with standard induction chemotherapy to patients with Diffuse Large B Cell Lymphoma (DLBCL) at high risk of central nervous system (CNS) relapse, as defined by the Lymphoma International Prognostic Index (CNS-IPI). Optimal timing of HD-MTX in relation to induction chemotherapy is unknown. A recent study suggested that HD-MTX intercalated with R-CHOP cycles was associated with increased toxicity and treatment delays without improvement in survival or CNS relapse compared with end of treatment MTX (Wilson et al 2020). This retrospective study evaluates the toxicities and treatment delays associated with HD-MTX administered on day 1 of cycles of chemo-immunotherapy. Methods: This single center retrospective cohort study included 45 patients (pts) with DLBCL with concurrent CNS disease or at high risk of CNS relapse who received HD MTX on day 1 of chemo-immunotherapy at our center. Data was abstracted from chart review and included variables describing clinical and treatment characteristics, time to MTX clearance, toxicities experienced and treatment delays. Results: 31 pts received HD-MTX on the day of R-CHOP chemotherapy, 6 pts received HD-MTX on the day of R-EPOCH (Rituximab, Etoposide, Doxorubicin, Vincristine, Cyclophosphamide, and Prednisone), and 8 pts received HD-MTX with R-MiniCHOP (dose reduced R-CHOP). Same day HD MTX with chemo-immunotherapy was associated with acute kidney injury (AKI; 17-25%), treatment delays >7 days (13-17%), and grade 2 mucositis (11-50%). The burden of toxicities was numerically higher in patients treated with R-EPOCH vs. R-CHOP (Table). Clinical outcomes are summarized in table below. Conclusions: In our heterogeneous population of pts, we describe that the incidence of toxicities and treatment delays experienced with same day HD-MTX are higher with R-EPOCH than with R-CHOP. Comparative studies with intercalated or end of treatment MTX will determine if the incidence of treatment delays, toxicities and de-escalations are higher with same day HD-MTX administration. [Table: see text]