Objectives: The aim of this research was to examine the sensitivity and specificity of transabdominal US, independently and together with certain clinical parameters, in the diagnosis of the stage of acute appendicitis in the pediatric population. Patients and methods: A retrospective study was conducted on 254 appendectomized children with a pathohistological finding of one of the stages of acute appendicitis, treated at the Department of Pediatric surgery, University Hospital of Split in the period from March 1, 2021 to March 1, 2023, who had medical histories stored electronically in BIS. Patients were compared based on demographic, clinical, laboratory and radiological data. In total, 151 children were examined by US diagnostic examination during that period. Results: US examinations were performed in 151 appendectomized children. US showed high sensitivity and variable specificity in the diagnosis of acute appendicitis, which is consistent with results from other published studies. Such variability of results is influenced by several factors such as the experience of the diagnostician, position of the appendix, perforation, etc. A higher diagnostic sensitivity was found in advanced stages of acute appendicitis (84.2%) than in initial stages (80.6%). The length of medical history, values of leukocytes, neutrophils and C-reactive protein (CRP) did not affect the higher positivity of detection of acute appendicitis by US, but the diagnostic sensitivity increased significantly by combining US and the mentioned parameters. The median age of children whose US was diagnostically positive was 11 years (IQR 9-14) and was three years younger than the median age of children whose US was diagnostically negative. The median age of these children was 14 years (IQR 11 – 16). Conclusion: Although the US is the most important preoperative diagnostic tool in the detection of acute appendicitis, the reliability of US diagnostics depends on many variables. They can be extremely variable and thus reduce the diagnostic reliability of US. Therefore, additional diagnostic tools are required. It was established that the use of additional laboratory parameters greatly increases the specificity of US diagnostics.