In South African blacks colorectal cancers are uncommon and adenomatous polyps are rare, despite major changes in diet and lifestyle in the large urbanized community of Soweto (population over 3 million). Factors favouring adenomas formation are changes in bowel function. Thus, stool weights, defaecation frequencies, oro-caecal transit times and whole-gut transit times in Sowetans approximate those of western populations. Moreover a measure of westernization of diet has occurred, the salient feature being a greatly reduced fibre intake (14 g daily). Factors thought to inhibit adenoma formation are physiological malabsorption of maize, lactose and fructose. Sucrose activity is significantly lowered compared with whites. With regard to diet, fat consumption is well below that of western societies. There are differences in colon cell proliferation, faecal short chain fatty acids, faecal pH and colonic microflora compared with white groups. Equivocal factors that may be involved in adenoma formation are time trends and genetic background. In the context of the above it is apparent that in South African blacks inhibiting factors outweigh promotive factors in adenoma formation.