Porcine lungs were macroscopically and microscopically examined at slaughter, with special regard to different stages of lesions similar to those caused by Mycoplasma hyopneumoniae. There was good conformity between the macroscopical and microscopical findings. In an extended abattoir survey, lesions were found in 4210 out of 4508 lungs examined. The majority of lungs with pleuritic lesions (274 out of 369) revealed by the extended examination were registered by the official procedure. No correlation between pleuritis and time for seroconversion, or with the levels of antibodies to Actinobacillus pleuropneumoniae, was found. Among lungs affected with pneumonic lesions (n = 3841), lesions similar to those caused by Mycoplasma hyopneumoniae were predominant (n = 3769). Only 15% of these lesions were revealed by official registration at slaughter. This figure is explained by the fact that only 35% of the infections were still active at the time of slaughter and that only ongoing lesions exceeding a certain magnitude were recorded according to the official regulations. By following the development of antibodies to Mycoplasma hyopneumoniae through the fattening period, the duration of the active infection was estimated to be approximately 12 weeks. Consequently, infections with Mycoplasma hyopneumoniae gained during the early fattening period will, in general, escape detection at slaughter.