Abstract A representative sample of 4800 slaughter pigs was taken at a large Danish abattoir. Blood samples were obtained, chronic pleuritis (CP) lesions were recorded and routine slaughter information (meat inspection data, sex, weight and herd identification) was noted. All sera were examined for specific antibodies against Actinobacillus pleuropneumoniae serotype 2 and 6 (AP2, AP6), Hemophilus parasuis and Influenza suis. A questionnaire was forwarded to the herds supplying pigs to the sample requesting information about the production system, herd size and vaccination programs. A total of 623 herds supplied pigs to the sample, yielding an average of 7.3 pigs per herd. Of the herd owners, 513 responded to the questionnaire. CP was detected in 28.0% of all carcasses. The serological survey revealed positive titers for AP2 and AP6 in 27.6 and 10.3%, for H. parasuis in 40.3% and for I. suis in 49.9% of the pigs, respectively. Corrected for confounding factors in a multiple logistic regression model, adjusted odds ratios for the serological risk factors with the strongest association with CP were AP2 = 9.21, AP6 = 1.72 and I. suis = 1.25. Atrophic rhinitis, decreasing weight at slaughter (reflecting age differences) and increasing herd size were also significantly associated with an increase in the risk of CP. H. parasuis, male (as opposed to female), fattening herd (relative to sow herd) and conventional herd (relative to specific pathogen-free) were not associated with an increased risk of CP. This study demonstrated that AP2 is probably the factor with the largest impact on the frequency of CP in Danish pigs for slaughter. It was calculated that 44% of all CP lesions in the sample could be attributed to this infection. However, I. suis is also of importance, mainly because of its high prevalence. It was estimated that 11% of all CP lesions could be explained by I suis.