DNA content was measured in 269 benign adenomas and 203 adenocarcinomas of the large bowel by flow cytometry. Nuclear suspensions were prepared by pepsin digestion of paraffin sections, and an adjacent section was graded histologically by one observer. Aneuploid adenomas included five of 134 (4%) showing mild dysplasia, 19 of 107 (18%) showing moderate dysplasia, and 10 of 28 (36%) showing severe dysplasia. The association between aneuploidy and grade of epithelial dysplasia was highly significant. This gives support to the adenoma (dysplasia)-carcinoma hypothesis. Carcinomas showing aneuploidy included 18 of 29 (62%) well differentiated, 92 of 144 (64%) moderately differentiated, and 19 of 30 (63%) poorly differentiated cases. There was no relation between aneuploidy and grade of carcinoma. These findings indicate an important biological difference between adenoma and carcinoma.