The poor prognosis of gallbladder cancer and the presence of high-risk populations make the identification of a screening test for this disease very desirable. As part of an ongoing case-control study of gallbladder cancer being conducted in Mexico City, Mexico, and in La Paz, Bolivia, blood specimens were sought from all patients with cancer of the gallbladder and on controls of similar age and sex undergoing upper abdominal surgery. Each sample was analyzed for carcino-embryonic antigen (CEA) and CA 19-9. Using the specimens from Bolivia, a serum CEA cutoff of 4.0 ng/ml yielded a sensitivity of 50.0% and a specificity of 92.7%, while a serum CA 19-9 cutoff of 20.0 units/ml yielded a sensitivity of 79.4% and a specificity of 79.2%. Using ROC curve analysis, the latter was a much better test than the former (p less than 0.05). Using the tests in series or in parallel did not substantively improve the results. The specimens from Mexico were used for validation purposes, and yielded very similar results. In conclusion, serum CA 19-9 and CEA are fairly good tests for discriminating patients with gallbladder cancer from patients with gallstones and no cancer, the former being a better test than the latter. These tests may be useful in identifying disease recurrences. In addition, if a sufficiently high-risk population could be identified, this could potentially become a useful screening test for this serious disease, allowing early intervention. However, additional data are needed prior to recommending this clinically.