PURPOSE: Most patients who finally diagnosed as the cecal and ascending colonic diverticulitis would complain pain on right low quadrant of abdomen. So many of them unfortunately would be performed emergency operation for presumed appendicitis. We are purposed to verify the usefulness of colonoscopy for the diagnosis and aimed to treat many patients with this disease conservatively. METHODS: We reviewed retrospectively the medical records of the 46 patients who diagnosed as the cecal and ascending colonic diverticulitis under admission at general surgery department during 4 years from January, 1997 to December, 2000. RESULTS: The mean age was 40.1 years and the male to female ratio was 1.2:1. Most common clinical manifestation was abdominal pain (46 cases) and nausea/vomiting (13 cases), fever/chill (4 cases) followed. On physical examination on abdomen, 26 patients had tenderness with rebound tenderness on right low quadrant and 18 patients showed only tenderness on right low quadrant. The mean peripheral WBC count was 10,600.9/mm2. Diagnostic tools were abdominal ultrasonography (34 cases), abdominal CT (13 cases), barium enema (8 cases) and colonoscopy (22 cases). The sensitivities of each modalities were 52.6%, 46.2%, 33.3% and 81.8% respectively. The respective mean hospital days depending on the treatment arms were as follows: 6.0 days of the conservatively treated group, 8.0 days of whom were operated as exploratory laparotomy with incidental appendectomy and 16.9 days of whom were operated as Right-hemicolectomy. 17 patients of 24 colonoscopy-done patients were recovered with conservative treatment, compared with only 7 patients of 22 colonoscopy-undone patients (P=0.0005). 2 cases of the conservatively managed groups were operated later due to recurrences (mean follow up periods=20 months). 2 operated patients had complications of postoperative ileus. CONCLUSIONS: Based on our study, the indications of colonoscopy are that in whom impressed clinically as acute appendicitis, on physical examination there be obvious tenderness on right low quadrant but obscure rebound tenderness, on peripheral blood smear the WBC counts range from normal to mild increased (<15,000/mm2), and on ultrasonography, appendix couldn't be detected or colonic wall show thickening. In patients who selected fit for indications, colonoscopy is safe and highly sensitive. We would manage these patients more conservatively, and may reduce their hospital stay.