Abstract:Objective: To retrospectively analyze the clinical characteristics of acute appendicitis (AA) and outcomes after appendectomy in≤5-year-old children. Methods: The clinical data of 538 children with AA aged five years or younger were collected retrospectively. According to the intraoperative findings and postoperative pathological examination, they were divided into complex appendicitis (CA) group (n=326) and simple appendicitis (SA) group (n=212). Clinical data of the two groups were compared and analyzed. Results: ①The median age of children in CA group was lower than that in SA group (3.4 years old vs.4.3 years old, P=0.000), while the admission temperature (38.4℃vs.38.1℃, P=0.000) and symptom duration (26 h vs.24 h, P=0.000) in CA group were higher than those in SA group, and the incidences of vomiting, diarrhea, rebound tenderness and abdominal distension in CA group were higher than those in SA group (all P<0.05).②The levels of white blood cell count, neutrophil count, lymphocyte count and procalcitonin level in CA group were higher than those in SA group before operation (P<0.05). In addition, white blood cell count was more effective in predicting CA, and the area under the ROC curve was 0.738 (95%CI=0.695-0.781).③The hospitalization time and postoperative antibiotic use time in CA group were longer and the intraoperative blood loss of children in CA group were greater than those in SA group (all P<0.05). Conclusion: The clinical manifestations of AA in children aged five years or younger are not typical. There were some differences in clinical manifestations, assay indicators and clinical outcomes between CA and SA children.