The efficacy of an inactivated aqueous vaccine against Mycoplasma hyopneumoniae was evaluated at two M hyopneumoniae-infected farrow-to-finish commercial farms (A and B) in Greece. In a prospective, randomised double-blind study, two groups on each farm received intramuscular doses of either the vaccine or the adjuvant when they were one and four weeks of age. The pigs were observed daily for clinical signs of disease; morbidity and mortality were recorded; and bodyweight was recorded at intervals. At slaughter, the lungs of the animals were examined and the chest cavities were examined for signs of pleuritis. No adverse reactions to the treatments were observed in any of the pigs. On farm A the vaccinated pigs were on average 6 kg heavier at slaughter, and on farm B they were on average 4 kg heavier; on both farms the average daily gain of the pigs was greater than that of the unvaccinated pigs. The prevalence and severity of enzootic pneumonia in the affected lungs were significantly lower in the vaccinated than in the unvaccinated pigs.