Porcine circovirus type 2 (PCV2) is the primary causative agent of porcine circovirus-associated diseases (PCVAD) in pigs. The open reading frame (ORF) 3 of PCV2 reportedly induces apoptosis and is associated with PCV2 pathogenicity. In this study, we first created an ORF3-null PCV2 mutant (muPCV2) by site-directed mutagenesis and demonstrated that the dimerized plasmid DNA of muPCV2 clone is infectious when injected intramuscularly (I.M.) into pigs. Subsequently, by using a well-characterized pig model we compared the pathogenicity of the muPCV2 and the wildtype PCV2. Thirty-one pigs were divided into 3 groups of 11, 10, and 10 each: group 1 pigs were each inoculated I.M. with PBS buffer as negative controls, group 2 pigs each with 200 microg of muPCV2 infectious DNA clone, and group 3 pigs each with 200 microg of wildtype PCV2 infectious DNA clone. Blood was collected prior to inoculation and weekly thereafter, and tested for PCV2 antibodies by ELISA and serum viral DNA loads by quantitative PCR. All pigs were necropsied at 35 days post-inoculation. The results showed that pigs inoculated with muPCV2 had a delayed seroconversion and lower serum viral load. However, there was no significant difference in the average scores of the histological or gross lesions or the amount of PCV2-specific antigen in tissues between wildtype PCV2- and muPCV2-inoculated groups. Thus, the data from this study do not fully support the conclusion of a previous report regarding PCV2 attenuation by abrogation of ORF3 although the results did show that ORF3 is dispensable for PCV2 replication in pigs.