Introduction. The course of opisthorchiasis is accompanied by clinically diverse symptoms and severe complications up to the development of cholangiocarcinoma. The role of oxidative stress in the development of liver fibrosis is not well understood. Aim. To determine the association of clinical manifestations and indicators of oxidative stress in the blood with liver fibrosis in patients with Opisthorchis felineus invasion.Materials and methods. We examined 103 patients with chronic opisthorchiasis and 51 practically healthy patients. All patients underwent general clinical examinations, esophagogastroduodenoscopy and ultrasound examination of the abdominal organs, elastometry to assess liver fibrosis using the METAVIR system, and the content of malondialdehyde, catalase and superoxide dismutase in blood serum was determined by the immunoassay method.Results and discussion. Asthenic-vegetative syndrome, pain in the right hypochondrium, articular syndrome, cytolytic and cholestatic syndromes, hepatomegaly and signs of chronic cholecystitis were more often detected in patients with invasion of Opisthorchis felineus and liver fibrosis F3-F4 according to METAVIR. The content of malondialdehyde in the blood was 296.5 ng/ml in patients with liver fibrosis F3-F4 according to METAVIR and 69.5 ng/ml in patients with liver fibrosis F0-F1 according to METAVIR (p < 0.001). The content of superoxide dismutase and catalase did not differ significantly in the groups of patients with liver fibrosis F0-F1 according to METAVIR and F3-F4 according to METAVIR, which indicated insufficient effectiveness of antioxidant protection.Conclusion. The revealed changes indicate the presence in patients with opisthorchiasis of a pronounced association between the severity of the clinical course, the development of biochemical cytolysis syndromes and the severity of liver fibrosis and oxidative stress, which may be a promoter of inflammation, cell DNA damage and carcinogenesis.