*Purpose: The aim of the current study is to investigate recent trends in mismatched organs utilized for orthotopic liver transplantation (OLT) and examine the impact of size mismatch on long-term graft survival amongst OLT patients with high and low model for end-stage liver disease (MELD) scores.

*Methods: The Organ Procurement and Transplant Network (OPTN) database was queried for all OLTs between 2007 and 2017. Size-mismatch was calculated using the body surface area index (BSAi) of donor to recipient. Patients were divided into 3 groups based on their BSAi (Undersized < 0.78, Matched 0.79 - 1.23, and Oversized > 1.24). Annual proportional trends of size mismatch were investigated. Recipients were stratified into two groups, MELD <20 and MELD >20. Kaplan-Meier survival curves and log-rank test were used to estimate and compare graft survival among these groups. Cox proportional hazards modeling was used to investigate predictors of graft survival at 5-years.

*Results: During this 10 year period, 65,015 OLTs occurred. Transplanted livers were undersized in 9.3 %, matched in 82.1% and oversized in 8.6%. Over the 10-year period, an increase of 0.2% and 0.86% was seen in the undersized and oversized groups respectively. MELD scores were <20 in 45.4% and >20 in 54.6% of recipients. After median follow up of 35 months, graft survival was 84.6% vs 87.5% vs 86.3% at 1 year, 80.5% vs 82.8% vs. 81.5% in 2 years and 71.4% vs 72.9% vs 74% at 5 years in undersized, matched and oversized groups respectively (log-rank test: p < 0.001). Subgrouping patients according to MELD showed a graft survival of 87% vs 89.3% vs 88.4% at 1 year, 81.2% vs 84% vs 83.3% in 2 years and 72.5% vs 73.6% vs 75.4% at 5 years for <20 MELD (log-rank test: p=0.032) and 83% vs 86% vs 84.8% at 1 year, 79.1% vs 81.2% vs 80.1% in 2 years and 70.6% vs 72.5% vs 72.5% at 5 years for >20 MELD in undersized, matched and oversized groups respectively (log rank test: p = 0.004). On multivariable analysis, independent predictors of toward graft survival were shorter warm ischemia time, younger age, a <20 MELD score, female sex, and an oversized liver.

*Conclusions: There has been an increasing trend in the usage of size mismatched OLTs over the last 10 years. Compared to undersized livers, recipients with oversized OLTs had improved graft survival, regardless of MELD score. Furthermore, oversized OLTs had similar or better long-term graft survival in comparison to matched OLTs.

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