A promoting effect of large bowel contents on colonic carcinogenesis as seen in the animal model is still incompletely explored in man. We investigated simultaneously deoxycholate absorption (as marker of colonic mucosal exposure to tumour promoting bile salt metabolites), mouth-anus transit time, and the ratio of anaerobic to aerobic bacteria in stool in 10 persons with colonic adenomas and in 10 age matched control subjects. We found that anaerobic/aerobic ratios and colonic deoxycholate absorption were higher in patients with colonic adenomas (p less than 0.002 and p less than 0.001) and that these parameters were clearly interrelated, which also applied to intestinal transit times and the anaerobic/aerobic ratios. These data are consistent with a promoting effect of the intracolonic environment on development of adenomas in man. Long term induction of a more aerobic colon flora and shortening of intestinal transit time may diminish bile-salt induced tumour promotion in adenoma patients.