Since the 1950s, breeding companies have been selecting commercial broilers for more rapid growth and higher meat yield, with a consequent increase in growth rate of about 5% a year. Rapid growth results from increased feed intake per unit of time and a higher metabolic rate and consequently leads to a higher demand for O2 . It has been estimated that 8% of 361, 000, 000 broiler deaths each year can be attributed to ascites. Ascites syndrome (AS) is an accumulation of fluid in abdominal cavity and the lay term ' water bally 'is an appropriate description of its appearance as affected chickens In 2002, it was estimated that these on-farm late grow-out mortalities cost more than 100 million dollar per year in the United States. In addition, 0.05% of all processing plant condemnations can be attributed to ascites . objective of this dissertation was to investigate the immunological, hematological and morphological changes associated with ascites in four broiler strains, and the genetic differences between the commercial broiler strains for the susceptibility to ascites.