Abstract.Calcium has been proposed to prevent colon cancer in subjects at risk for this tumour. This effect is supposed to be due at least in part to binding the bile acids to calcium, making them insoluble and harmless. To evaluate the effects of oral calcium supplementation on intestinal bile acids, 19 patients with adenomatous colonic polyps were supplemented with 35.5 mmol Ca2+daily for 12 weeks. Duodenal bile, 24‐h feces and 24‐h urine were collected betore and at the end of the 12‐week period. In duodenal bile proportional concentration of cholic acid increased (38 ± 4 vs. 51 ± 3%,P<0.001), whereas that of chenodeoxycholic acid decreased (35 ± 3 vs. 25 ± 2%,P<0.01). Total fecal bile acid excretion increased (950 ± 126 vs. 1218 ± 137 μmol 24 h‐1P<0.01), with proportional concentrations of the main primary and secondary bile acids remaining the same. Cytolytic activity of fecal water, measured by the degree of lysis of erythrocytes by the water, decreased (45 ± 8 vs. 30 ± 7%,P<0.05). Total excretion of calcium increased as expected from the supplementary dose. It is concluded that calcium supplementation markedly affects intestinal bile acids and lytic activity of fecal water and that, in view of similar results during 1‐week calcium supplementation in young healthy subjects, these effects remain constant over at least 3 months and occur both in healthy persons and in patients at increased ris