Abstract: Postweaning multisystemic wasting syndrome (PMWS) is a disease of nursery and fattening pigs characterized by growth retardation, paleness of the skin and dyspnea. It has been demonstrated that PCV2 is the cause of PMWS. The expression of the disease is dependent on the co-infection with PCV2 and other factors or immunostimulation. In this study, 32-day-old postweaning piglets without antibody to PCV2 were inoculated intranasally with PCV2. The effect of immunostimulation on the induction of PMWS was investigated by immunization with keyhole limpet hemocyanin(KLH) emulsified in incomplete Freunds adjuvant. Sixteen piglets were distributed to four groups,each with four: control, PCV2, PCV2+KLH and KLH. The results showed that the 2 groups inoculated with PCV2 showed growth retardation. But the clinical signs of pigs in the group inoculated alone with PCV2 were not clearer than those in group with PCV2+KLH. Pigs inoculated with PCV2 and immunostimulated with KLH had increased rectal temperature, and one of these pigs developed wasting and died in day 12 after challenge. PCV2 viremia was found from 4 days after challenge to the end of the experiment. PCV2 nucleic acid was found in the lung and lymph node from the pigs of two groups inoculated with PCV2 by PCR. Meanwhile, the pigs in the groups inoculated with KLH alone or control had no clinical sign and pathogen. It confirmed that PCV2 is the primary viral infectious agent of PMWS.