This study determines the infiltration rate of proximal and distal margins of resection in patients operated on for gastric cancer at the Istituto Nazionale Tumori of Milan. Two hundred and eighty-five proximal margins and 286 distal margins were reviewed, and the incidence of infiltration was related to the length of grossly tumor-free edge, the location, site, size and gross appearance of the tumor, degree of invasion of the gastric wall, histologic type, and status of perigastric lymph nodes. Infiltration occurred in 7.3% of oral margins of transection and in 2.6% of aboral margins. Except for the degree of invasion of the gastric wall, no correlation was found among the infiltration rate and the above parameters. In fact, the incidence of infiltration of the proximal edge was significantly higher (6.4+ vs. 0.8%, p less than 0.01) when the tumor penetrated the serosa or spread beyond it than when the lesion was confined to the mucosa, submucosa, or muscular layer. With reference to the length of margin of resection, it is noteworthy that no involvement was found when cranial distance between the lesion and line of transection was equal to or greater than 6 cm. Proximal or distal infiltration for a distance greater than 3 cm did not occur in patients with lesions confined to the mucosa, submucosa, and muscularis. This data should provide the surgeon with a rational basis for assessing the extent of resection when performing gastrectomy for cancer.