Serum CA19-9 levels have been measured in 34 patients with asymptomatic colorectal cancer, 39 age and sex matched subjects with healthy colons (as assessed at full colonoscopy) and 55 patients known to have liver metastases from primary colorectal cancers. In subjects with asymptomatic cancer the median CA19-9 was 12.75 U ml-1 (0.0-280.7 U ml-1), in the healthy controls the median CA19-9 was 12.80 U ml-1 (0.0-88.9 U ml-1) and in those with liver metastases was 62.5 U ml-1 (4.8-458.0 U ml-1). Levels were significantly higher in patients with metastatic disease than in patients with asymptomatic tumours or the healthy controls, however there was no significant difference between the asymptomatic group and the controls. Using an upper limit of normal of 37 U ml-1, the sensitivity of CA19-9 was 60.3% for the detection of colorectal cancer with liver metastases but only 17.6% for asymptomatic cancer. Serum CA19-9 estimation is of no value as a means of screening for asymptomatic colorectal cancer.