Hantavirus cardiopulmonary syndrome (HCPS) is caused by new world hantaviruses, among which Andes hantavirus (ANDV) is endemic to Chile and Southern Argentina. The disease caused by ANDV produces plasma leakage leading to enhanced vascular permeability and has a high case fatality rate (35%), mainly due to respiratory failure, pulmonary edema and myocardial dysfunction, hypoperfusion and shock. Host sociodemographic and genetic factors might influence the course and outcome of the disease. Yet, they have not been thoroughly characterized.To evaluate sociodemographic factors as risk factors in severity of HCPS.Study period: 2004-20013, attending in eight collaborative centers, etiological diagnosis was performed by serology or molecular biology, mild and severe HCPS were compared.139 Chilean patients were analyzed, 64 (46%) with severe disease among which 12 (19 %) died.European ethnicity had 5,1 times higher risk than Amerindian ethnic group to develop a severe HCPS, greater seriousness that was also associated with an urban residence.It was observed that ethnicity and type of residence were significant risk factors for HCPS severity. Hypotheses explaining these findings are discussed.