Several studies have demonstrated a relationship between mucosal carbonic anhydrase (CA) isoenzymes, particularly CA II, and cancer of the large intestine. Recent work has suggested the potential usefulness of fecal CA assay for colorectal cancer screening. This clinical study examined the accuracy of fecal CA II as a marker of adenocarcinoma of the colon and rectum. An enzyme-linked immunosorbent assay was used to measure CA II in urine, serum, and stool samples from 31 colorectal cancer patients and 26 control subjects. An immunochemical fecal occult blood test was also performed in all study participants. Urine and serum CA II were similar in the two study groups. However, both the prevalence and the mean level of fecal CA II in the cancer patients were significantly higher than those in the control group. The detection rate for CA II in the stool was 65 per cent for the cancer patients versus 4 per cent for the control population. The fecal CA II test was similar in sensitivity and specificity to the immunochemical fecal occult blood test (65 vs 48%; 96 vs 100%). Measurement of fecal CA II might be useful in screening for colorectal cancer.