Antibodies to O. viverrini in the sera of people from endemic and non-endemic areas were investigated using indirect ELISA technique. For the patients from the endemic area, 92.8% who passed eggs in the stool were found to be positive for O. viverrini antibody. In addition, 46.5% of the people who did not pass eggs in the stool were also found to have low titer of O. viverrini antibody. On the other hand only 2.4% of the people from the non-endemic area with other intestinal parasite infections were found to have O. viverrini antibody in their sera. It was concluded that positive reaction of O. viverrini antibody is not cause by cross-reaction with other parasites but low liter of antibody is probably due to low-level or past infection. There is a positive correlation between the titer of O. viverrini antibody and intensity of infection as indicated by number of eggs excreted per milligram of feces. Patients with a few O. viverrini eggs in feces, but biopsy-proved-cholangiocarcinoma had very high titer of antibody.