Hydatidosis is endemic and enzootic disease in Chile. Previous national studies of seroprevalence showed a rate of 135 x10(5), corresponding to approximately 17,000 infected persons. The prevalence of human hydatidosis shows a declining tendency in recent years except in Regions IV, VI, and IX of Chile. Region IV presents higher rates of infection in bovine, ovine, and caprine livestock. A human study was conducted to evaluate the seroprevalence in Region IV (Coquimbo), zone extended between 29º 02' and 32º 16' South latitude. A total of 4,632 blood samples were collected in the provinces of Elqui, Limari, and Choapa. The sample size correspond to approximately 3% of the rural population of the area in the study. Whole blood samples were collected using filter paper and blood was obtained from the tip of middle finger. ELISA IgG test was used to determine antibodies against E. granulosus. The samples were analyzed in two laboratories at the same time. The antigen was prepared from ovine hydatid cysts. The fluid was collected aseptically, centrifuged at 12,000 g in one laboratory and in the other one, fraction B (Oriol antigen) was used. A seroprevalence of 2,500 x 10(5) was determined. This rate is significantly higher than the national one 1.65 x 10(5), and the regional rate 6.73 x 10(5) notified in 2004. The rural areas surveyed in the study included mainly cattle farmers. The seroprevalence observed in Limari and Choapa provinces were higher than in Elqui province. The differences may be due to differences in the economic and laboral make-up of the provinces. Further studies are necessary to identify human patients and confirm the infection using additional immunological diagnosis (e.g Western blot) and clinical evaluation with imaging characterization of the patients