AIM: To assess histochemical expression of KL-6 and its clinicopathological significance in carcinoma of the ampulla of Vater.METHODS: Ampullary carcinoma tissues were collected from 38 patients who underwent pancreatoduodenectomy or local resection. Tissues were subjected to immunohistochemical analysis using KL-6 antibody.RESULTS: Positive staining of ampullary carcinoma cells was observed in 26 (68.4%) cases. Staining was not found in the surrounding non-cancer regions of the ampullary tissues. Remarkable KL-6 expression was observed in invasive carcinoma cells in pancreatic and duodenal tissues and in metastatic carcinoma cells in lymph nodes. Positive KL-6 expression was related to lymph node metastasis (P = 0.020), pancreatic invasion (P = 0.016), duodenal invasion (P = 0.034), and advanced stage of TNM clinical classification (P = 0.010). Survival analysis showed that positive expression of KL-6 was related to a poorer prognosis (P = 0.029).CONCLUSION: The aberrant expression of KL-6 mucin is significantly related to unfavorable behaviors of carcinoma of the ampulla of Vater.