Background: Preoperative histologic grading was used to decide the length of the margin of resection in adenocarcinoma of colon and rectum.
Objectives: The aim of the present study was to determine the accuracy of preoperative histologic grading by comparison with postoperative histologic grading.
Material and Method: Preoperative and postoperative histologic grading of patients with adenocarcinoma of the colon and rectum in the department of surgery, Phramongkutklao Hospital between January 1999 and October 2004 were collected and compared together using percentage and chi-square test.
Results: 260 patients were included in the present study. 47.3% of all cases had inaccurate preoperative histologic grading. 43% of all cases had worse differentiation. 52.7% of all cases had the same differentiation. Only 4.3% of all cases had better differentiation.
Conclusion: Preoperative histologic grading was not accurate and could not be used in deciding the length of the margin of resection.

Keywords: Adenocarcinoma, Rectum, Colon, Differentiation, Grading