Evidence is presented that Borna disease (BD) virus, which is known to cause encephalopathy in horses, sheep, and a broad range of experimental animals, or a related agent, can infect man and may induce mental disorders. BD virus-specific antibodies could be demonstrated in 4-7% of sera (depending on origin) from more than 5000 psychiatric or neurological patients from Germany, U.S.A. and Japan. Antibodies from seropositive patients reacted with a BD virus-specific protein translated by RNAs which were transcribed from a cDNA clone obtained from BD virus-infected tissues. When the cerebrospinal fluid from three seropositive patients was inoculated into rabbits or rabbit embryonic brain cell cultures, evidence was obtained that suggests the presence of BD virus or a related agent.