Objectives: To evaluate the accuracy of sonography in diagnosing acute appendicitis in patients with Alvarado score 4–7.Methods: This is a retrospective cross-sectional study being performed in Namazee hospital affiliated with Shiraz University of Medical sciences during a one year period from 9/2007 to 9/2008. We evaluated all patients with Alvarado score 4-7 and divided them in two groups: those with Ultrasound study prior to surgery and those without any imaging modalities for diagnosis of AA. The demographic information, histopathology, physical examination, laboratory data, sonography report and histopathological reports of patients were gathered. Results: A total of 238 patients had Alvarado scores 4-7 including 160 males and 78 females. 110 patients did not have any imaging and 128 had undergone sonography before operation. Ultrasound had overall sensitivity of 75 %, specificity 69.2 %, PPV 88 %, NPV 46.1% and accuracy of 73.6 %. Negative appendectomy rate was20.9 % and 23.4 % in those without sonography and inpatients with sonography respectively, with a higher rate in females.Conclusion: Ultrasound is more useful when the patient is female and the result of sonography is positive; however, it is not reliable when the result is negative and maybe other diagnostic modalities such as CT scan can help us in better diagnosis of Acute Appendicitis.