In comparison with copro-antigen detection in opisthorchiasis reported previously by Wat- wiengkam et al [17], urinary antigen level was approximately 25 times lower than copro-anti- gen at infection intensities of 1–100 EPG. The explanation for this discrepancy is probably due to the fact that OV adult worms secrete antigen directly into the bile duct before being passively swept into the gastrointestinal tract and voided in the feces as copro-antigens. By contrast, using our urinary antigen detection method, the parasite antigen may be sequestrated or trapped in the liver or in other tissue, and diffused into the circulation before being excreted via the kidneys with urine. Therefore,