Since 1996, the U.S. swine industry has seen disease outbreaks characterized by reproductive failure at early gestation and/or neurologic disorders in sows and young pigs, which are commonly referred to as "Porcine reproductive and neurologic syndrome (PRNS)". A previously unrecognized small enveloped RNA virus of approximately 50 nm in size was repeatedly isolated from field cases of PRNS and tentatively named "Virus X". The disease and/or lesions of PRNS were reproduced in pregnant sows and young caesarian-derived-colostrum-deprived (CDCD) pigs experimentally infected with Virus X and the virus was re-isolated from those pigs, fulfilling Koch's postulates. Both polyclonal and monoclonal antibodies specific for Virus X were produced. The polyclonal antibody did not cross react with any known viral pathogens for swine except for ruminant pestiviruses (i.e., bovine viral diarrhea virus and border disease virus) on immunofluorescence test and Western immunoblotting. Polyclonal antibodies raised against the ruminant pestiviruses cross-reacted with Virus X to a degree. However, Virus X could not be recognized by pan-BVD virus monoclonal antibodies. Sequence and peptide analysis of Virus X for 5' UTR and NS3 regions indicated that the virus is phylogenetically separate from any known pestiviruses. Furthermore, Virus X could be differentiated from the ruminant pestiviruses by laboratory procedures employing the monoclonal antibody (15A2-3E9) produced in this study. In conclusion, a previously unrecognized swine pestivirus is responsible for the newly identified disease.