International audience ; This study was set up to get more insights in the severity and relevance of porcine circovirus type 2 (PCV2) infections in Dutch fattening farms in an endemic PCV2-situation with no clinical signs of post-weaning multisystemic wasting syndrome (PMWS). In part A of the study, in total 29 commercial fattening farms with varying percentages of pneumonia and pleurisy at slaughter were examined. Blood samples were collected at random by cross-sectional sampling; 10 in the age of 10-12 weeks, 10 at the age of 16 weeks and 10 blood samples at the end of the finishing period (20 - 22 weeks of age). Serum samples were examined for the presence of PCV2 IgM and IgG antibodies and for antibodies against other porcine lung pathogens. In part B, 8 "high"- and 8 "low" herds were selected. The 8 "high" herds were defined as herds having high percentages of lung lesions (pneumonia) at slaughter, and the 8 "low" herds had low percentages of pneumonia at slaughter. For both the "high" and "low" herds, 3 pigs showing signs of respiratory distress were selected for necropsy (n=48). Lung tissue samples were examined post-mortem for macroscopic and histopathological lesions, and for the presence of bacteria and viruses. The results of part A showed that, pigs at 16 weeks of age with IgM antibodies against PCV2 had a lower probability of having pleuritis at slaughter (OR 0.34, P<0.000). Pigs in the age category of 20 - 22 weeks, and with IgM antibodies against PCV2, also had a lower probability of having pneumonia at slaughter (OR 0.29, P=0.032). In part B lobus apicalis pneumonia, PCV2 in macroscopically unaffected lungs, , and swine influenza viruses were all found significantly more often in "high" than in "low" pigs at autopsy. High PCV2 DNA loads (>10 PCV2 DNA copies/mg) were found in lungs of 14 (58%) "high", and in 7 (29%) of the "low' pigs (P=0.13). In 11 of the 19 affected lungs from "high" pigs, high PCV2 DNA loads were found in combination with one or more other lung pathogens, while this was ...