Objective：To analyze the expression of cyclooxygenase-2（COX-2） in patients with familial adenomatous polyposis and to discuss the possible role of COX-2 in adenoma formation and cancerization. Methods： The adenoma specimens were obtained from 10 FAP patients （2001-2003）. Different sizes （≤0.5 cm and ≥1 cm） of adenomas and cancerous adenomas were subjected to Envision immunohistochemistry for analysis of COX2 protein expression. Results： COX2 was mainly detected in epithelial cells, with some in interstitial cells. When ≤0.5 cm, COX-2 expression in adenomatous epithelial cells was stronger than in normal epithelial cells （P〉 0.05）; the same was true when ≥1.0 cm, but with statistical significance （P〈0.01）. The expression of COX-2 in different sizes of adenomas was significantly different（P〈0.01）, with that in ≥1.0 cm adenomas stronger than that in ≤0.5 cm adenomas. Expression of COX-2 in cancerous adenoma was stronger than that in normal epithelial cells （P〈0.01） and epithelial cells of ≥1.0 cm adenomas （P〉0.05）. Conclusion： COX-2 may be a stimulative factor in the progression of FAP and may participate in adenoma formation and cancerization. Selective inhibition of COX2 may be an effective strategy for preventing FAP