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A total of 29 520 animals, from 164 batches of pigs belonging to an identical number of herds, were involved in the study. The considered population of pigs were limited to the region of north-eastern Poland involving six voivodeships. From each herd, samples of blood were collected to evaluate the antibody titres to Mycoplasma hyopneumoniae, porcine reproductive and respiratory syndrome virus, Aujeszky's disease virus and swine influenza virus. At an abattoir, the lung lesions of each batch were scored and the enzootic pneumonia-like lesion average value was calculated. Lesions, indicative of enzootic pneumonia, were found in 57.8% of the lungs. For all lungs, the enzootic pneumonia-like lesion average value was 1.74, ranging from 0.42 to 3.56 among the 164 batches. In the examined pig population, 57.8% were considered suffering from swine respiratory disease, the majority of the affected pigs came from the Podlaskie (21.7%) and Greater Poland (17.25%) voivodeships. In the most affected voivodeships, 88.37% and 85.16% of the farms were considered as disease-susceptible for Greater Poland and Podlaskie, respectively. The findings indicate that, in pigs in north-eastern Poland, the major pathogens causing pneumonia-like lesions are Mycoplasma hyopneumoniae (68.9%) and porcine reproductive and respiratory syndrome virus (44%). [ABSTRACT FROM AUTHOR]