Vaccines against M. hyopneumoniae and PRRSV are often administered at the same time. The administration of the two vaccines mixed in one syringe (simultaneous use) would save labour and increase animal welfare by reducing the number of injections. The aim of this study was to verify under field conditions, if the simultaneous administration of the two vaccines, would be comparable to the administration of the individual vaccines. 1000 piglets were divided into five groups and were either non vaccinated or vaccinated against M. hyopneumoniae and/or PRRSV-EU-type. Body weight, local reactions, morbidity and mortality were measured and clinical examinations were carried out. Blood samples were analysed serologically for the presence of antibodies against PRRSV and M. hyopneumoniae by ELISA. M. hyopneumoniae-induced lung lesion scoring (with a maximum score of 55 points) was performed at the slaughterhouse. In the 1000 pigs included in this study, no clinically relevant local or systemic reactions were observed following vaccination. No significant differences in average daily weight gain (ADWG), mortality, clinical parameters or in the prevalence of pneumonia and pleuritis at the slaughterhouse were detected between the different study groups. Significant differences were observed for the serological results, with M. hyopneumoniae antibody positive pigs in the vaccinated groups being significantly higher than in the non-vaccinated groups. Regarding the severity code of the lung lesions of the affected lungs, the lung scores from the simultaneously vaccinated pigs were significantly lower (0.92) than those from non-vaccinated pigs (1.77). Due to the low infectious pressure of both PRRSV and M. hyopneumoniae on this farm, we have to conclude that efficacy of the vaccines could not adequately be tested.