A prospective study of 170 patients with suspected appendicitis was performed to assess the value of sonography in detecting the normal and abnormal appendix. The wall thickness (normal, less than or equal to 3 mm), compressibility of the appendix, and echogenicity of surrounding fat were the primary criteria used to determine the status of the appendix. Of 60 patients who underwent surgery, appendicitis was proved in 45. The remaining 110 patients who did not have surgery, contacted by telephone at the end of the study, had no clinical follow-up evidence of acute appendicitis. A normal appendix was clearly identified in 102 (82%) of 125 patients without acute appendicitis. The sensitivity of sonographic examination in detecting appendicitis was 93%, the specificity was 94%, and the accuracy was 94%. The predictive value of a positive test was 86%; that of a negative test was 98%. Ruptured appendicitis was predicted in all cases (11/11). Sonography is useful in detecting acute appendicitis and can clearly show the normal appendix more frequently than previously reported.