Sera from 295 horses in the USA were examined by an indirect immunofluorescence assay and Western blot assays to determine the prevalence of Borna disease virus infection. Eight (2.7 per cent) of the samples were positive in both assays, and 18 (6.1 per cent) were positive only in the Western blot assay. The indirect fluorescence titres ranged from 1:20 to 1:80 of antibodies recognising the virus-specific antigen from Borna disease virus-infected cells. The purified virus-specific proteins isolated from infected rat brains were recognised by positive equine serum samples after immunostaining by a Western blot technique. Information obtained from the owners about the history of the seropositive horses revealed that they were either clinically normal or had a pathological diagnosis of disease unrelated to Borna disease. This is the first report of the detection of antibodies to Borna disease virus in horses in the USA. The disease may be more widespread in a subclinical form, with very long incubation periods, and may not necessarily be restricted to historically endemic areas.