Fifty-two surgically removed large bowel specimens and 3 colonoscopically removed polyps with mucosal carcinomas were studied. Among 36 frank colorectal carcinomas, six (17%) had a focus of adenomatous remnant. The average size of carcinomas without adenomatous remnant was 56.0 mm and of those with adenomatous remnant was 44.3 mm. Four (67%) of 6 submucosal carcinomas and all of 4 mucosal carcinomas showed an adenomatous remnant. The average size of submucosal carcinomas was 17.7 mm and that of mucosal carcinomas was 16.3 mm. The smaller and less advanced the carcinoma, the more likely it was to show an adenomatous remnant, suggesting that carcinomas arise in adenomas and destroy surviving benign tumour as they grow. Coexisting adenomas were seen in 19 of 46 neoplasm-bearing patients (41%) and 1 of 9 patients (11%) without neoplasms. Average sizes of adenomas with moderate and mild atypia were 13.0 mm and 8.2 mm respectively, which were smaller than the sizes of submucosal and mucosal carcinomas. Although the numbers were small, these findings support the concept of adenoma-carcinoma sequence. Twenty-one flat elevations were collected during the study. Ten were neoplasm (9 adenomas and an early carcinoma), ten were metaplastic polyps and one was a histologically normal mucosal protrusion. The atypism of 10 flat adenomas increased with increasing size, as with ordinary adenomas. As 10 of 55 adenomas (18.2%) were flat adenomas, which are difficult to detect during routine colonoscopic examination, colonoscopists should make every effort to discover flat adenomas, which seem to play an important role in the adenoma-carcinoma sequence.