We conducted a retrospective cohort study of children who had chronic fascioliasis in the highlands of Peru to determine triclabendazole treatment effi cacy. Children passing Fasciola eggs in stool were off ered directly observed triclabendazole treatment (>1 doses of 10 mg/kg). Parasitologic cure was evaluated by using microscopy of stool 1-4 months after each treatment. A total of 146 children who had chronic fascioliasis participated in the study; 53% were female, and the mean ± SD age was 10.4 ± 3.1 years. After the fi rst treatment, 55% of the children achieved parasitologic cure. Cure rates decreased after the second (38%), third (30%), and fourth (23%) treatments; 17 children (11.6%) did not achieve cure after 4 treatments. Higher baseline egg counts and lower socioeconomic status were associated with triclabendazole treatment failure. Decreased triclabendazole effi cacy in disease-endemic communities threatens control eff orts. Further research on triclabendazole resistance and new drugs to overcome it are urgently needed.