infection in the pre-patent period where eggs are yet to be detected. Fur- thermore, the results confirmed that there was no cross reactivity with other helminth parasites endemic to the area since the levels of antigen in subjects with O. viverrini infection alone or with mixed infection with other parasites (i.e. S. stercoralis, echinostomes, hookworm, minute intestinal fluke and Taenia) were not different. However, further tests of urine samples from non-endemic areas of opisthorchiasis are required to ensure the lack of this observed cross reactivity.