Objective: Appendicitis is a common surgical emergency, for which sonography is widely used to assess the presence of absence of appendicitis. The objective of this study was to identify factors associated with the diagnostic accuracy of sonography in patients with a suspected case of acute appendicitis. Methods: A retrospective review was conducted in all patients who were assessed for acute appendicitis (with sonography) in two emergency rooms of a large hospital. The sonography result was compared with the pathological report in patients who underwent surgery under suspicion of an acute appendicitis. Results: A multivariate analysis revealed that operator (radiologist or resident), time of the sonogram, site (hospital), and body mass index were independent variables significantly influencing the sonographic result. Correctly diagnosing suspected appendicitis with sonography is 2.2 times more likely when performed by a radiologist compared to a resident. Conclusions: Sonography is widely used in diagnosing appendicitis in the emergency room. In this study, the probability of sonography being correctly diagnosed decreases during on-call hours or when conducted by a resident in patients with suspected appendicitis. Furthermore, increasing body mass index decreases the probability of a sonogram being correct when conducted by a resident compared to a radiologist.