Objectives: Ulnar collateral ligament reconstruction (UCL-R) surgeries have increased significantly in amateur and professional baseball pitchers. While high school (HS) showcase participation has been considered an injury risk factor, limited data corroborating this association exists. Despite concern that showcase participation in amateur athletes may lead to higher risk of future injury, there is a scarcity of UCL injury-specific research on elite amateur pitchers using reliably recorded pitch velocity and other performance data rather than self-reported metrics. The objectives of this study were to examine the age at which elite high school pitchers began to achieve high fastball velocity, specifically greater than 90, 92, and 95 mph (≥90/92/95 mph), as well as to examine the relationship between showcase participation and future need for UCL-R during their professional career. We hypothesize that pitchers achieving fastball velocities ≥90/92/95 mph at younger ages are more likely to undergo UCL-R earlier in their professional careers compared to pitchers not achieving these velocity thresholds. Additionally, we hypothesize that pitchers participating in high showcase volumes are more likely to undergo UCL-R compared to pitchers participating in fewer showcases. Methods: Demographic, HS showcase performance, and injury data from pitchers selected in the first 5 rounds of the Major League Baseball (MLB) Draft (2011-2020) were gathered from publicly available databases. Demographic data collected for each pitcher included date of birth, height/weight/body mass index at time of MLB Draft, position (left- vs. right-handed pitcher), MLB Draft year, MLB Draft round and overall pick, high school state/country, college (if attended), whether pitcher entered professional baseball directly after HS, U.S. region, MLB organization which drafted pitcher. Performance data were collected for each pitcher exclusively from PG (www.perfectgame.org) and PBR (www.prepbaseballreport.com) events participated in during their HS careers prior to MLB draft. Performance metrics included: peak fastball velocity at HS showcases, age at which a pitcher achieved ≥90/92/95 mph at a HS showcase, age at which slider was first thrown in a HS showcase, peak slider velocity in a HS showcase. UCL injury data included: age at UCL-R, specific amateur or professional level when injured, and recovery time (in months). Continuous and categorical variables for pitchers undergoing UCL-R and those not undergoing UCL-R were compared, and multivariable analysis was performed using logistic regression. Standard deviations of ±1 SD were used to define “early” versus “late” UCL-R subgroups after normal distribution was confirmed (Shapiro-Wilk test, p = 0.183). Trends in time were evaluated using linear regression. The significance of trends over time was evaluated using linear regression. Data analysis was performed using SPSS Version 27 (IBM Corp). A two-tailed P