Abstract Background Overtriage has been observed among pediatric patients in emergency departments (EDs) under five-level acuity pediatric triage systems. This study aimed to investigate the causes of overtriage and to provide suggestions for future amendments to such systems. Methods This study has a retrospective follow-up design in which 42,000 pediatric patients who were admitted to the emergency department of the largest medical center in Taiwan between January and December of 2010 were recruited. The study variables included patient demographics, chief complaints, individual vital signs (i.e., blood pressure, heart rate, body temperature, respiratory rate, and O2 saturation), triage level, ED final disposition, ED expenses, and total medical expenses. A logistic regression model was applied to explore the causes of overtriage and the effectiveness of a modified acuity system. Results Approximately 13.6% of the pediatric ED visits were upgraded to acuity level 1 based on vital signs that included heart rate (97.3%). The strength of the trend association (odds ratio) between decreasing acuity urgency (from levels 1 to 5) and hospitalization increased from 0.73 (95% CI: 0.72-0.75) with the Ped-TTAS to 0.57 (95% CI: 0.55-0.59) with the modified Ped-TTAS, which downgraded acuity levels by excluding the weighting of vital signs. Further validation was accomplished by comparing the trend association between decreasing acuity urgency and total medical expenses (Ped-TTAS: β=-0.13; modified Ped-TTAS: β=-0.18). Conclusions Heart rate is prone to be affected by emotional responses among pediatric patients in certain specific age groups. Appropriate revisions of the pediatric triage system are suggested.