The objectives were to evaluate the effectiveness of ultrasonography, computed tomography, and physical examination for diagnosing acute appendicitis with analyzing their accuracies and negative appendectomy rates in a clinical rather than research setting. A total of 2763 subjects were enrolled. Sensitivity, specificity, positive predictive value, and negative predictive value and negative appendectomy rate for ultrasonography, computed tomography, and physical examination were calculated. Confirmed positive acute appendicitis was defined based on pathologic findings, and confirmed negative acute appendicitis was defined by pathologic findings as well as on clinical follow-up. Sensitivity, specificity, positive predictive value, and negative predictive value for ultrasonography were 99.1, 91.7, 96.5, and 97.7 per cent, respectively; for computed tomography, 96.4, 95.4, 95.6, and 96.3 per cent, respectively; and for physical examination, 99.0, 76.1, 88.1, and 97.6 per cent, respectively. The negative appendectomy rate was 5.8 per cent (5.2% in the ultrasonography group, 4.3% in the computed tomography group, and 12.2% in the physical examination group). Ultrasonography/computed tomography should be performed routinely for diagnosis of acute appendicitis. However, in view of its advantages, ultrasonography should be performed first. Also, if the result of a physical examination is negative, imaging studies after physical examination can be unnecessary.