Butyrate is produced in the colon by fermentation of dietary fibre and induces apoptosis in colon adenoma and cancer cell lines, which may contribute to the protective effect of a high fibre diet against colorectal cancer (CRC). However, butyrate is present in the colon together with unconjugated bile acids, which are tumour promoters in the colon. We show here that bile acids deoxycholate (DCA) and chenodeoxycholate (CDCA), at levels present in the colon, gave a modest increase in cell proliferation and decreased spontaneous apoptosis in AA/C1 adenoma cells. Bile acids significantly inhibited the induction of apoptosis by butyrate in AA/C1 cells. However, the survival-inducing effects of bile acids on AA/C1 cells could be overcome by increasing the concentration of sodium butyrate. These results suggest that dysregulation of apoptosis in colonic epithelial cells by dietary factors is a key factor in the pathophysiology of CRC.