In a prospective study 34 Peruvian fascioliasis patients were followed up for 3 months after treatment with praziquantel. The rapid sedimentation of faeces was demonstrated to be the most appropriate parasitological technique for the diagnosis of Fasciola hepatica infections in man compared to the MIF concentration or the examination of duodenal fluid by Enterotest. After therapy with different doses of praziquantel only 21% of the patients stopped excreting worm eggs although 17 patients had been treated repeatedly. During the period of observation all patients presented a decreasing number of symptoms. It is concluded that praziquantel is not very active against F. hepatica in man.