The occurrence of inclusion body hepatitis (IBH) on 11 properties in Australia is described. The 6 Victorian cases occurred between 1967 and 1973 and were identified as incidental findings accompanying other diseases or occurred as ill-defined, minor economic disease episodes. In well-managed flocks the extra mortality attributable to IBH was as low as 1.8% which represented a 50% increase in total mortality. The 1973 epidemic in New South Wales on 5 related farms resulted in flock mortality of 8.4% to 32.3% and an average mortality of 19.5% of 814,000 birds. For uninfected flocks of 307,000 birds the average mortality was 8.5%. Higher mortality in male flocks was statistically highly significant as was lowered efficiency of feed conversion. Growth rate in both sexes was depressed by about 0.15 kg in 70 days and heavy downgrading at processing occurred. Secondary infections were important. The cardinal features of IBH were fatal necrotising hepatitis with Cowdry type-A intranuclear inclusion body formation; secondary bacterial infections were also important causes of loss.