AbstractReactive oxygen metabolites (ROM) have been postulated to contribute to the development of various carcinomas, including colon cancer. Indeed, the effects of ROM scavengers are being tested for chemoprevention of adenocarcinoma of the colon. However, there has been no evidence to indicate that high levels of ROM are indeed present in cancerous tissue. In this study, we used a chemiluminescence probe to estimate ROM levels in cancerous and neighboring noncancerous colonic tissues from seven patients with colon cancer. Cancerous tissues contained significantly (p<0.05) more luminol‐enhanced chemiluminescence (4,808±2,282 counts/min/mg protein) than neighboring noncancerous tissues (2,175±1,111). The addition of an ROM scavenger, catalase (2, 4, and 8μg/ml), to the tissue suspension inhibited chemiluminescence produced by both noncancerous (—74%,—85%, and—71%) and cancerous (—11%,—61%, and—53 %) tissues. This study shows that colonic cancerous tissue contains high levels of ROM, which may play an important role in the pathogenesis of colon cancer.