Randomized controlled trials (RCT) on adjuvant chemotherapy for gastric cancer published in the West and Japan were reviewed. Although several small trials showed positive data, adjuvant chemotherapy for curatively resected gastric cancer has been thought to be ineffective in western countries. Results of Japanese RCTs also have not become evidence of its benefit. Despite this, suggestive data by non-predefined subset analyses of old RCTs have been misread as definitive evidence of benefit because of less understanding of clinical statistics in Japan. As a result most Japanese patients have received postoperative adjuvant chemoimmunotherapy. Recently understanding of clinical trial has spread gradually and well designed RCTs with sufficient sample size have been reported. First of all we have to determine the efficacy of adjuvant chemotherapy by carefully designed RCT using surgery alone arm as control.