From 1987 to 1991, a seroepidemiologic survey for antibodies to hantaviruses, leptospira, rickettsiae, and Borrelia was conducted in selected Italian population groups. In the mountainous areas of northeastern Italy, the prevalence of antibody to hantaviruses, as detected by indirect immunofluorescent antibody (IFA) assay, was 7.1%, 4.8%, 4.3%, and 4% in 265 forestry workers, 82 rangers, 395 farmers, and 75 hunters, respectively. Among 299 Alpine soldiers, the prevalence was lower (0.7%). Of those with Hantaan antibody, the reactivity pattern using Hantaan, Puumala, and Fojnica viruses suggested a prevalence of antibody to Hantaan virus, with titers reaching levels of 128. The presence of leptospiral antibodies (by microagglutination test), which included the prevalence of antibodies to Leptospira icterohaemorrhagiae, L. bratislava, and L. saxkoening serotypes, was observed in 10-12% of the farmers and forestry workers in these Alpine mountain regions. Only a few sporadic clinical cases of leptospirosis have been reported from these regions. Antibodies to Borrelia burgdorferi (by IFA) were observed in 19% of the rangers and forestry workers, with lower values in farmers (10%) and hunters (8%). These data suggest the presence of a large number of asymptomatic infections with B. burgdorferi and the leptospires in the densely wooded areas of the Alpine Italian regions. Furthermore, the recent identification of a case of Hantaan acute nephropathy in a man living in the mountainous northeastern area of Italy confirms the presence of hantavirus in the Italian Alpine zones, especially those near the Slovenian border.