We report on 2 patients from Siberia suffering from an infection with the parasite Opisthorchis felineus. The unusual course of their disease pretended in case 1 an eosinophilic leukemia and in case 2 a malignoma of the gallbladder. The Opisthorchiasis is endemic in large areas of Asia and Russia. Humans acquire the infection by eating raw fresh-water fish. Symptoms are nonspecific, but detection of eosinophilia in travellers or residents of endemic areas should induce analysis for specific antibodies against Opisthorchis species. Opisthorchiasis is known to be a precursor of cholangiocarcinoma. Malignoma which was initially suspected could be excluded in both cases and the patients were cured by oral administration of Praziquantel, 40-75 mg/kg body weight for 1 day in 3 divided doses.