In searching for a screening test to identify a population at high risk for large bowel cancer, methane production was measured in 45 patients with colorectal carcinoma compared with 67 individuals who served as a control group. There was no significant difference in methane production between the colorectal cancer patients and the control group (37.8 and 25.4% respectively). Within the colorectal cancer group 54% of the males were methane producers compared with 19% of the females (P = 0.03). There were no differences according to disease stage. In view of these results, we see little value in using expired air methane concentration as a screening test for large bowel cancer.