A matched case control study was conducted in order to determine the association between previous cholecystectomy and the development of large bowel cancer. One hundred and sixty men and 145 women presenting with large bowel cancer were studied. One control was selected for each case matched according to age, sex and date of admission. The medical records for each patient in the study were reviewed for evidence of previous cholecystectomy. No association was found among male patients, but for women an estimate of relative risk of 2.7 was obtained for patients who had had previous cholecystectomy. This association was largely a consequence of the fact that 10 female patients with rectal cancer had had a previous cholecystectomy compared with only of their controls. It is possible that carcinogenic breakdown products of bile salts are most concentrated in the distal portion of the large bowel, but it is difficult to explain why the association should be confined to women only.