Abstract: Six-week-old healthy piglets were experimentally inoculated with porcine reproductive and respiratory syndrome virus (PRRSV) or porcine circovirus type 2 (PCV2) alone or in combination (PRRSV and PCV2). Realtime PCR was used to quantitatively measure proinflammatory cytokines, i.e. IL-1β, IL-6, IL-8 and TNF-α, mRNA expression in peripheral blood mononuclear cells (PBMC) from piglets. After infection, the PRRSV- and PCV2-infected group tended to have increased expression of IL-1β, IL-6, IL-8 and TNF-α mRNA, showing significant upregulation of gene expression for IL-6 and IL-8 in the PRRSV group and for IL6, IL8 and TNFα in the PCV2 group, in comparison to the control group. The PRRSV and PCV2 coinfected group showed increased gene expression for IL8 and TNFα only, but both of them showed significant upregulation as compared to the control group. Furthermore, the PRRSV and PCV2 coinfected group showed decreased levels of IL1β, IL6, IL8 mRNA, while the levels of TNFα mRNA were significantly increased, in comparison to the singly infected group. These results suggest that the excessive expression of TNFα may play an important role in the synergistic pathogenesis of PRRSV and PCV2.