In this study, pigs were injected with a nonreplicating human adenovirus type 5 vector expressing porcine interferon-alpha (Ad5-pIFN-alpha) and then challenged with porcine reproductive and respiratory syndrome virus (PRRSV) to determine whether the presence of increased levels of IFN-alpha would decrease viral replication and/or disease. Groups of 10 pigs each were inoculated with Ad5-pIFN-alpha and not challenged, Ad5-pIFN-alpha and challenged with PRRSV 1 d later, or inoculated with a control adenovirus that does not express IFN-alpha (Ad5-null) and challenged 1 d later with PRRSV. IFN-alpha levels in all pigs inoculated with the Ad5-pIFN-alpha were elevated the day of challenge (1 d after inoculation), but were undetectable by 3 d after inoculation in the pigs that were not challenged with PRRSV. Pigs inoculated with Ad5-pIFN-alpha and challenged with PRRSV had lower febrile responses, a decreased percentage of lung involvement at 10 d post-infection, delayed viremia and antibody response, and higher serum IFN-alpha levels as a result of PRRSV infection, compared to pigs inoculated with Ad5-null and challenged with PRRSV. These results indicate that IFN-alpha can have protective effects if present during the time of infection with PRRSV.