Liver fluke infection caused by Opisthorchis viverrini (OV) is a major public health problem in parts of East Asia. In Thailand, an estimated 6 million are people infected with OV. Experimental and epidemiological evidence strongly implicate OV infection in the etiology of cholangiocarcinoma (CCA). Studies have shown 10-20% of OV-infected persons have advanced periportal fibrosis (HBA) that progresses to CCA. OV infection may cause inflammation of the bile ducts that results in DNA damage of the epithelium and subsequent malignant transformation. The primary objective of this study is to determine if in-vitro levels of inflammatory cytokines induced by crude antigen extracts of OV are associated with the hepatobiliary abnormalities known to progress to CCA in OV-infected adults aged 20 to 60 years of age. The secondary objective is to determine if in-vitro levels of inflammatory cytokines induced by crude antigen extracts of OV in adults with hepatobiliary abnormalities change 12 months after treatment with praziquantel and if these changes are associated with persistent advanced fibrosis (grade 2 or 3) as determined by ultrasonography. The primary outcome measure is elevated levels of inflammatory cytokines (such as TNF-alpha) induced by OV infection are associated with a 50% greater risk (OR = 1.50) of having advanced fibrosis (grade 2 or 3) as determined by ultrasonography. The secondary outcome measure is elevated levels of inflammatory cytokines 12 months after treatment for OV infection are associated with a 50% greater risk (OR = 1.50) of maintaining the advanced fibrosis that is known to progress to CCA over this time period. District health outposts reporting greater than or equal to 20 percent prevalence of OV infection among adults will be identified, whereupon males and females, 20 to 60 years of age, will be screened for OV infection. Study sites in Thailand will be districts of Ban Pai, Chonnabot, Muncha Khiri and Kok Pochai, regions along the drainage of the Chi River, in the vicinity of the regional capitol, Khon Kaen city. A recruitment ceiling of 20,000 individuals will be established to screen for infection with OV, with 6,000 enrolled in the observational and longitudinal parts of the study. OV-infected individuals will receive an ultrasound examination that will be graded as either HBA positive (advanced fibrosis) or HBA negative (non-advanced fibrosis). Over a 24 month period, 1,116 OV-infected individuals who are HBA positive will be recruited. The first 484 OV-infected individuals with HBA will be age and sex matched with OV-infected individuals from the same region who are HBA negative. Blood samples will be drawn from all volunteers and used in immunological evaluations of OV infection in relation to HBA status. This will constitute the observation stage of the study. The 1,116 OV-infected individuals with HBA will be followed for an additional 12 months, with an ultrasound exam and a blood draw. This will constitute the longitudinal stage of the study. All volunteers will then be treated for OV infection with praziquantel.