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Background and Aims Accurate sample size calculation is essential to the validity of results from randomized controlled trials (RCT), especially in the context of null findings. Currently, no systematic evaluation of sample size calculation errors exists that focuses on the regional anesthesia literature.Methods Among 11 RCTs published in 2025 in Regional Anesthesia and Pain Medicine (RAPM), we reviewed sample size calculation descriptions to assess appropriateness and describe common errors. Calculations were subsequently revised and compared to the original sample sizes.Results Among 11 included RCTs ( table 1), we identified 5 potentially erroneous sample size calculations. Errors were mostly related to: (1) arbitrary selection of effect sizes without adequate justification, (2) discrepancies between assumed and actual outcome means and variances, and (3) inappropriate use of two-sided testing when one-sided would have been sufficient. Some studies assumed opioid consumption means that were substantially higher than those ultimately observed, leading to miscalculated sample sizes. Others applied overly large effect sizes without support from existing data, resulting in underpowered trials that failed to detect clinically meaningful differences. Overpowered studies resulted when two-sided testing was inappropriately used to calculate sample size for a superiority trial.Abstract OP43 Table 1Reported and corrected sample size calculations for five RCTs with potentially erroneous sample size calculationsConclusions Sample size calculation errors are not uncommon in the regional anesthesia literature, and can critically undermine trial validity or lead to inconclusive results. Additionally, enrollment of more participants than necessary may expose them to procedural risks without additional scientific benefit. Improved rigor in sample size justification, including transparent reporting of assumptions and sensitivity analyses, is needed to enhance the quality and reliability of future RCTs.