A survey of hydatid disease has been carried out at the Royal Prince Alfred Hospital over a ten-year period from 1964 to 1974. It would appear that there has been no change in incidence in hydatid disease in the last 25 years. Fifty per cent of the hydatid cysts dealt with in this series occurred in the liver, and 30% were in the lung. Forty per cent of patients with lung hydatid disease were found to harbour hydatid cysts in the liver as well, while 24% of those with hydatid disease in the liver were found to have co-existent lung cysts. Twenty per cent of patients with pulmonary hydatid cysts presented with urgent complications, while only 7% of those with liver hydatid cysts presented in this way. Twenty-four per cent of the patients in this series were born outside Australia, most of the rest having been born and having lived in New South Wales. In this State, the distribution of hydatid disease remains the same as it was in 1925. Of the serological tests, the Casoni intradermal reaction was positive in 67% of cases, while the complement-fixation reaction was positive in only 52%. Seventy-eight per cent of these patients came to operation; 50% were found to have viable cysts at operation. Morbidity was high from chest infection, prolonged drainage from the site of the cyst and subphrenic abscess. Since few viable cysts were found in patients over the age of 60 years, there is a strong case for conservatism in the treatment of elderly patient with an asymptomatic calcified hydatid cyst. The hydatid-dependent mortality rate in this series was 1-8%. Over a mean follow-up period of five years, in 5% of these patients, there was a proven recurrence of hydatid disease requiring further operation.