The seroprevalence of cystic echinococcosis in north-western Mongolia was determined in a cross-sectional study of 334 semi-nomadic pastoralists in their summer grazing areas. Overall, 5.2% of the subjects were found to be strongly seropositive for antibodies to Echinococcus granulosus antigen B by ELISA. Only 10% of the adult subjects (aged > or = 15 years) had heard of the disease and only 5% recognized hydatid cysts in their livestock (in contrast to the neighbouring province in China, where 76% had seen and recognized cysts in the organs of slaughtered animals). The close association between Mongolian pastoralists, their dogs and livestock is likely to encourage ready transmission of the parasite in a region where home slaughter is practised and offal for various scavengers, including domestic dogs. Further information is needed to determine the prevalence of infection in other intermediate hosts and in dogs and the clinical importance of the disease in this remote region.