An outbreak of hydropericardium syndrome characterized by distension of the pericardium with fluid and severe involvement of liver and kidneys appeared in Pakistan in broiler chicks in 1987. The disease usually affects chicks aged 3-5 weeks old and causes 20-60% mortality. The disease was first reported in Karachi, Pakistan, in Autumn of 1987 and then spread throughout the country. An agent (tentatively designated adenovirus) was isolated from liver, heart and kidneys of the affected birds. The agent was highly pathogenic when inoculated into susceptible healthy broiler chicks and could be grown in chicken kidney cells and embryonated eggs. Being filterable through seitz filter pad, it produced intranuclear inclusion bodies in chicken kidney cells and haemagglutinated rat red blood cells. These characteristics of the isolated virus were similar to those of adenovirus reported by other workers.