The pathology of human fascioliasis was studied before and after bithionol treatment using light and transmission electron microscopy. Fine needle biopsies were taken from five patients, with established fascioliasis, before and after drug administration. By light microscope the pathology of human fascioliasis was similar to that reported in experimental fascioliasis. The ultrastructural picture revealed bile ductular hyperplasia, fibrosis of portal tracts, widening of the interhepatic spaces by many microvilli and dilated Disse space with collagen fibres. Bile ductular hyperplasia may be the initial factor to fibrinogenesis, which subsequently enhance the development of the microvilli on the surface of the hepatocytes. Both light and electron microscopic studies revealed regression of the picture of fascioliasis to normal after bithionol treatment with no sign of toxicity on the liver.