A total of 231 flat colorectal neoplasias were investigated at two disparate geographical regions (Stockholm and Tokyo). Of the 141 flat neoplasias seen in Tokyo, 24.8% had HGD, 7.0% intramucosal carcinoma and 9.9% invasive carcinoma. On the other hand, of the 90 flat mucosal neoplasias seen in Stockholm, 13.3% had HGD, 1.1% intramucosal carcinoma and 1.1% invasive carcinoma. The differences between flat adenomas with HGD, intramucosal carcinomas and invasive carcinomas in Japanese patients were significantly higher (p < 0.001) than in Swedish patients. While the causes responsible for this geographic difference remains unclear, possible influences of ethnicity and/or of environmental factors were advanced.