Abstract: In 2017, a mass mortality of the 60- to 80-day-old young blue-shell chickens freely raised in a forest of a mountain region in Zhuolu County of Hebei Province was observed. According to the clinical symptoms and pathological-anatomical changes, the disease was preliminary diagnosed as suspected hydropericardium-hepatitis syndrome （HHS）. In order to perform the confirmative diagnosis, the clinical samples were recovered from the diseased and dead chickens with typical clinical symptoms and were used for the isolation, cultivation and identification of bacterial and viral pathogens. Furthermore, the virulence of the isolates was assessed in 10-week-old SPF chickens. While three isolates of Escherichia coli were obtained, no typical clinical symptoms and death were observed in the challenged SPF chickens with the isolated strains. Round virus particles with diameters of 60-80 nm were observed by electron microscope; the erythrocyte agglutination test result demonstrated that the viral isolate was able to agglutinate the rat red cells; the SPF chickens challenged with the viral isolate expressed typical pathological-anatomical changes of HHS; the causative agent was identified as adenovirus type 4 by agar diffusion test.