The transitional mucosa (TM) adjacent to carcinomas of the large bowel shows histologic and mucin histochemical changes that may indicate premalignant change and may be of prognostic value after radical resection. In this study, 10 anterior resection specimens from patients with carcinomas of the rectum and rectosigmoid were used to compare the nuclear DNA content in TM with those in cancer tissue and with those in nontransitional mucosa (N-TM; i.e., uninvolved mucosa remote from tumors showing normal histologic and mucin histochemical features). The nuclear DNA content was assessed using DNA image cytometry on Feulgen-stained sections. As compared with N-TM, crypts in TM contained greater numbers of cells, were elongated, and were more likely to be branched with marked sialomucin secretion, accompanied by a marked reduction in the normal sulfomucin content. The mean nuclear DNA content in the upper, middle, and lower thirds of crypts was significantly higher in TM than in N-TM, and the nuclear DNA content in TM exhibited no correlation to that in tumors. The results suggest that TM adjacent to carcinomas of the rectum and rectosigmoid has higher proliferative activity, providing further evidence that TM may be an unstable premalignant change.