Clinicopathologic features and prognostic significance of duodenal invasion were studied in a retrospective study on 593 patients who underwent gastrectomy for adenocarcinoma in the antrum. The patients were grouped into three, according to the histologic extent of duodenal invasion: Group A (80 patients), obvious invasion beyond the pyloric ring; Group B (61 patients), invasion up to the pyloric ring; and Group C (452 patients), no evidence of duodenal invasion. Five-year survival rates in Groups A, B, and C were 7.9%, 31.6%, and 57.6%, respectively (P less than 0.001). Cox's regression analysis showed that duodenal invasion is an independent prognostic factor in cases of a gastric antrum carcinoma. Gastric cancer with duodenal invasion (Groups A and B) most often was infiltrative and the incidence of serosal invasion, lymphatic and vascular invasion, and lymph node metastasis was high. Duodenal invasion was direct through submucosal or subserosal layers or through submucosal lymphatics.