PURPOSE: To compare the accuracy of initial clinical diagnosis with that of ultrasonography(US) in diagnosingappendicitis and to determine its effect on the care of patients in a community hospital. MATERIALS AND METHODS: One hundred and fifty patients with suspected appendicitis were prospectively examined with US. Prior to this,they had been divided into three groups on the basis of clinical judgement regarding diagnosis and their treatmentplan: Group I : high suspicion (probability > 75%) - urgent surgery indicated ; Group II : equivocal (probability25 - 75%) - in hospital for observation, Group III : very unlikely (probability < 25%)- discharge without furthertesting. In order to determine the validity of these groupings, we calculated the sensitivity, specificity, andaccuracy of sonographic examination, and the likelihood ratios of initial clinical diagnosis. RESULTS: USrevealed a sensitivity of 92%, specificity of 90%, positive predictive value of 95%, negative predictive value of85% and overall accuracy of 91%. The initial clinical impression showed a sensitivity of 70%, specificity of 82%,positive predictive value of 89%, negative predictive value of 58%, and overall accuracy of 74%. Among 79 patientsin the high clinical risk category (Group I), the sensitivity, specificity and accuracy of US were 96%, 89%, and95%, respectively. Among 71 patients in the low- and intermediate- clinical categories (Groups II and III), thesensitivity, specificity, and accuracy of US were 83%, 90%, and 87%. The likelihood ratios were 3.9 in Group I,0.52 in Group II, and 0.15 in Group III. In 32 of 150 patients (21%), the findings of US led to changes in theproposed management plan. CONCLUSION: The overall accuracy of US in the diagnosis of appendicitis wasstatistically superior to that of the clinician's initial impression (p<0.05). In addition, US played an importantrole in making decisions regarding the treatment plan.