INTRODUCTION: In Brazil, Mato Grosso (MT) has the highest number of hantavirus cardiopulmonary syndrome cases. Our study aimed to identify anti-hantavirus antibodies in the sera of patients from Sinop, MT, presenting with acute febrile illness. METHODS: A retrospective analysis of data for 198 sera samples assessed using enzyme-linked immunosorbent assay (ELISA) was conducted. RESULTS: Immunoglobulins G (IgGs) against the hantavirus nucleoprotein were found in 13.6% of the tested sera. No sample had immunoglobulin M (IgM) antibodies to hantavirus. Seropositivity occurred mainly in female residents in urban areas who worked around the household. CONCLUSIONS: Our findings suggest circulation of hantavirus in Sinop.