SUMMARYTo determine the frequency of Strongyloides stercoralis antibodies by means of the enzyme linked immunosorbent assay (ELISA) in Chile, in 2001Chile, in -2003 blood samples of patients of two psychiatric hospitals and 172 of healthy individuals (doctors, nurses and paramedicals) of these institutions, and 1,200 serum samples of blood donors of Northern region (Arica and Antofagasta), Central region (Valparaiso and Santiago) and Southern region (La Union) were collected. ELISA showed positivity of 12.1% in psychiatric hospitalized patients, none (0%) in the health personnel and 0.25% in blood donors (p < 0.05). Only in blood donors of Arica (1%) and La Union (0.5%) the ELISA test was positive suggesting that strongyloidiasis is focalized in determinate zones of the country. In Chile, human infections by S. stercoralis are endemic with very low frequency in apparently healthy individuals and high prevalence in risk groups such as the mentally ill hospitalized patients. KEYWORDS:Strongyloides stercoralis; Seroepidemiology; Strongyloidiasis; Chile.Human infection by the nematode Strongyloides stercoralis is especially endemic throughout tropical and warm temperate regions of the world 3,11 . In other geographical zones or in industrialized countries, strongyloidiasis still represent a clinical-epidemiological problem focalized in institutions for the mentally ill patients 10 . Scarce epidemiological information about strongyloidiasis in Latin-American countries is available. Epidemiological studies in selected studied populations of Brazil, Peru, Colombia and Venezuela showed frequency of 13.0% 14 , 8.7% 20 , 2.3% 8 and 1.4% 7 respectively. In Chile, in 1983, a mortal case of disseminated strongyloidiasis in a hospitalized psychiatric 53 year-old man was reported 17 . Enteroparasitic infections surveys carried out in psychiatric hospitalized patients of the V geopolitical region of Chile in 1985 and 1990 showed percentages of S. stercoralis infections of 11.6 (57/490) 5 and 7.0 (16/229) 9 respectively. In 2000, in a psychiatric hospital of Santiago, Chile it was observed that 20 patients presented elevated blood eosinophiles count. Coproparasitological examination of seven stool samples of each patient permits the detection of larvae of S. stercoralis in four of them 15 .The laboratory diagnosis of strongyloidiasis by using commonly stool examination methods such as formol-ethyl acetate techniquesroutinely used in Chile -is considered of low sensitivity, so the identification of S. stercoralis infected individuals is difficult 4 . Immunodiagnosis of S. stercoralis infection has been attempted with several serological tests. Enzyme linked immunosorbent assay (ELISA) permits the detection of serum specific IgG 1 , and can be used as an epidemiological tool to determine the prevalence of strongyloidiasis in a given region or population 18,19,20 . In Chile, ELISA test to detect S. stercoralis serum antibodies against an alkaline protein extract of the filariform larvae of S. venezuelensis was standardized 16 .The...