Histologic sections from 781 nonprotruding colorectal neoplasias (adenomas and early carcinomas) recorded in Sweden and in Japan were reviewed by the same observer, using strict histologic criteria. Low grade dysplasia (LGD) was present in 82.8% (299/361) of the nonprotruding neoplasms recorded in Sweden but in only 42.6% (179/420) of those recorded in Japan. On the other hand, high grade dysplasia (HGD) was found in 42.4% (178/420) of the nonprotruding colorectal neoplasias in Japanese patients but in only 14.1% (51/361) of those in Swedish patients. Whereas 15.0% (63/420) of the nonprotruding neoplasms in Japan were intramucosal carcinomas (IMCs) or submucosal carcinomas (SMCs), only 3.0% (11/361) of those reviewed in Sweden were IMCs or SMCs. Thus in Japan the lesions were histologically more "advanced" (significantly more lesions had HGD) and more "aggressive" (significantly more lesions were IMC or SMC) than in Sweden. The cause(s) of that phenomenon remains elusive. The term nonpolypoid adenoma connotes a group of dysplastic lesions with different histologic phenotypes. Interestingly, similar phenotypes were recently found in overt colorectal carcinomas in both Sweden and Japan. Experimental and clinical findings indicate that nonprotruding colorectal adenomas are closely associated with colonic lymphoid nodules, suggesting that the lymphoid-associated colorectal mucosa might be the birthplace of many nonprotruding adenomas (and consequently of nonprotruding early colorectal carcinomas).