Postweaning Multisystemic Wasting Syndrome (PMWS) is a recently emerged multifactorial clinical disease of swine that affects nursery and growing pigs and PCV2 is believed to be the main aetiological agent of the disease. PMWS has emerged in Ireland as a clinical entity in pig herds so there was therefore a need to determine the extent of PCV2 infection and associated disease in the pig population of Ireland. The prevalence of PCV2 in Ireland was determined by testing a percentage of porcine sera for antibodies to PCV2 using the IPMA method; this method is based on PCVfree PK15 cell cultures infected with PCV2 virus. Ten serum samples (where available) from fifty randomly selected herds (426 sera samples) were tested, of the fifty herds only four were negative to PCV2. However these four herds were all hobby herds (less than 12 pigs). In most cases of the ten samples tested per herd eight to ten of the sera samples were positive for PCV2 antibodies indicating that there is widespread seroconversion to PCV2 in Ireland. To determine the extent and emergence of the disease a study on suspect herds was performed. This study commenced on the 1st January 2004 and was completed on the 1st January 2005. This study involved working in collaboration with local pig veterinarians and regional veterinary laboratories so that any herd that developed any syndromes typical/similar to PMWS were reported and suspect herds tested for the presence of PCV2 antigen. Tissue samples (especially lymph nodes) were taken from necropsied pigs (approximately 4 per herd) and tested by using a combination of indirect fluorescent antibody staining of cryostat tissue sections, Abstract immunohistochemsitry and histopathology. A total of 30 suspect herds in Republic of Ireland were tested over this twelve month period. 24 of these herds were found to be positive for PMWS and 6 herds negative. In Northern Ireland, 31 herds were tested and 14 of these herds were PMWS positive, based on PCV2 antigen levels. PCV2 was isolated from diseased and non-diseased pigs in Ireland. The entire genome of PCV2 was amplified by PCR sequenced. Complete genomes of PCV2 were then aligned and a phylogentic analysis was performed. ROI and NI isolates were closely related to each other displaying 97-100% overall nucleotide homology. An amino acid (aa) alignment was performed on the two major Open Reading Frames, ORF1 and ORF2, as these encoded the major proteins, Replicase and Capsid, respectively. The majority of aa changes observed between PMWS positive and negative isolates occurred within ORF1. A detailed longitudinal study was also carried out. A total of 4 herds were involved in the study, 2 herds from ROI which were reported as positive for the disease, one negative control herd from ROI and one positive herd from NI. The study involved monitoring approximately 5 litters from each of the 4 herds for a total of 10 weeks from birth in order to elucidate any co-factors that may lead to PMWS. Serum samples were tested for PCV2 antibody titre and PPV antibody, tonsil and faeces samples were tested for the presence of other viruses such as enterovirus 1 and 2, reovirus and adenovirus and faecal swabs for bacterial organisms such as haemolytic E. coli , Campylobacter spp. and Salmonellae spp. From the study it could be concluded that there was no distinguishable coinfection of PCV2 with any of the infectious organisms mentioned between positive and negative animals and herds.