This review presents data on the characterization of Borna disease virus (BDV) and its potential as a possible causative agent in humans. The isolation of: (i) BDV-specific cDNA clones that encode various BDV-specific proteins and (ii) partially purified virus particles led to the conclusion that the viral genome consists of negative-sense, single-stranded RNA. The organization of the BDV-specific RNA species appears to be a nested set of overlapping subgenomic RNA transcripts. Furthermore, evidence is presented that BDV can infect humans and may cause certain psychiatric and neurological disorders. This concept is supported by: (i) the finding of virus-specific antibodies in sera of patients with neuropsychiatric diseases and (ii) results obtained during attempts to isolate BDV or a BDV-related agent from the cerebrospinal fluid of seropositive patients.