Background: Only 5–10% of emergency medical services (EMS) patients are children, and most pediatric encounters are low-acuity. EMS chart review has been used to identify adverse safety events (ASEs) in high-acuity and high-risk pediatric encounters. The objective of this work was to evaluate the frequency, type, and potential harm of ASEs in varied acuity pediatric EMS encounters. Methods: This cross-sectional study evaluated pediatric (ages 0–18 years) prehospital records from 15 EMS agencies among three states (Colorado, Connecticut, and Rhode Island) between November 2019 and October 2021. Research associates used a previously validated tool to analyze electronic EMS and hospital records. Adverse safety events were recorded in six care categories, grouped into four levels for analysis: assessment/diagnosis/clinical decision-making, procedures, medication administration (including O2), and fluid administration, and defined across five types of ASEs: Unintended injuries or consequences, Near misses, Suboptimal actions, Errors, and Management complications (UNSEMs). Type and frequency of ASEs in each category were rated in three harm severities: Harm Unlikely, Mild/Temporary, or Permanent/Severe. Three physicians verified ASEs determined by research associates. Frequency of ASEs and harm likelihood are reported. Results: Records for 508 EMS patients were reviewed, with 63 (12.4%) transported using lights and sirens. At least one clinical intervention beyond assessment/diagnosis/clinical decision-making was documented for 183 (36.1%, 95% CI: 31.8, 40.4) patients. A total of 162 ASEs were identified for 112 patients (22.1%, 95% CI: 18.5, 25.7). Suboptimal actions were the most frequent UNSEM (n = 66, 40.7%; 95% CI: 33.1, 48.3). For ASEs, (n = 162), the most frequent associations were with procedures 39.5% (95% CI: 32.0, 47.0) or assessment/diagnosis/clinical decision making, 32.1%, (95% CI: 24.9, 39.3). Among care categories, fluid administration was associated with significantly more UNSEMs (58.1%, 95% CI:53.8, 62.4). Most ASEs were determined to be 'Harm Unlikely' 62.4% (95% CI: 54.4, 70.4), with assessment/diagnosis/clinical decision making having significantly fewer ASEs with documented harm (22.4%, 95% CI: 10.7, 34.1) compared to other care categories. Conclusion: Over 20% of pediatric EMS encounters had an identified ASE, and most were unlikely to cause harm. Most frequent ASEs were likely to be associated with procedures and assessment/diagnosis/clinical decision-making. [ABSTRACT FROM AUTHOR]