Field trials were used to assess the efficacy of an inactivated vaccine against hydropericardium syndrome in broiler chickens. A single vaccination at 10 to 12 days old was effective for the control of the syndrome; mortality in the vaccinated birds was 0.52 per cent compared with 5.34 per cent in unvaccinated birds kept at the same premises. Vaccination was also effective when carried out in the face of an outbreak; mortality in the vaccinated infected birds was 2.33 per cent compared with 10.27 per cent in unvaccinated infected birds. The data indicate that a formalinised vaccine prepared from the liver of experimentally infected birds could be used for the control of hydropericardium syndrome.