Prognostic value of microscopical resection-line involvement of the proximal and distal resection lines was examined in patients undergoing potentially curative resection in a multicentre trial with strict quality control. Tumour-positive resection lines were seen in 41 of the 699 evaluable patients (5.9%). Resection-line involvement was associated with T stage (P < 0.001), N stage (P = 0.003), tumour location (P < 0.001) and tumour differentiation (P < 0.001). Presence of resection-line involvement was also associated with significantly worse survival. Together with T3 and T4 stage (relative risk (RR) 2.04), N(+) stage (RR 4.02) and tumour differentiation (RR 1.33), resection-line involvement (RR 2.28) was also an independent prognostic factor for survival. Survival in patients with resection-line involvement was comparable with patients with positive cytology. In patients undergoing potentially curative resection for gastric cancer, peri-operative frozen-section examination should be mandatory, especially in those with poorly differentiated, signet ring cell or anaplastic tumours, with tumours covering the entire stomach and T3 or T4 tumours. In case of tumour-positive margin(s) at final histology, re-laparotomy could be considered if achieving tumour-free resection lines seems realistic.