To verify the role of sonography in screening of acute appendicitis in patients admitted to an infectious disease unit for suspected acute infectious enteritis.One hundred eighty consecutive patients (102 male and 78 female; age range, 5-72 years; mean age, 31 years) admitted for suspected infectious enteritis or typhoid fever were prospectively studied with abdominal sonography within 48 hours after admission. None of the patients had peritoneal irritation. Forty-six patients (25%) had white blood cell counts of more than 10,000/mm3 (range, 10,300-18,000/mm3). The diagnosis of acute appendicitis was made when a detectable appendix with an anteroposterior diameter of greater than 7 mm could be seen on sonography.Eleven (6%) of 180 patients had thickened appendixes (anteroposterior diameter range, 7-14 mm); 2 of them had periappendiceal abscesses. Four (36%) of 11 patients with acute appendicitis had high white blood cell counts. All sonographic diagnoses of acute appendicitis and periappendiceal abscesses were confirmed at surgery. Sonography ruled out acute appendicitis in 169 patients. In all of them, clinical and sonographic follow-up excluded the diagnosis of acute appendicitis. Normal appendixes were shown on sonography in 38 (22%) of 169 cases and were not detectable in 131 (78%) of 169.Sonography of the appendix is a useful method for early assessment of acute appendicitis in patients thought to have enteritis or typhoid fever.