The clinical course of 20 patients with 22 thoracic hydatid cysts operated on from 1957 to 1984, was reviewed. Follow-up extended to 27 years (mean, 11.6 +/- 1.5 years [+/- the standard error of the mean]) and was 90% complete. Most patients originated from countries in the Mediterranean region; 5 were native Canadians. The diagnosis was suspected in all but 3 patients preoperatively. Fourteen patients had 16 primary lung cysts, 4 had infected liver cysts with intrathoracic extension, 1 had synchronous unruptured liver and lung cysts, and 1 had a thymic cyst. Nine cysts were intact, whereas 13 had ruptured preoperatively. A variety of surgical procedures was performed, including eight wedge resections and nine lobectomies. There were no early deaths, and perioperative complications were infrequent. One patient with a hepatic cyst that had ruptured into the right pleural space and right lower lobe died at 7 months of massive echinococcosis of the liver and intraabdominal sepsis. There has been no evidence of thoracic recurrence in any of our long-term survivors, although 1 patient had undergone enucleation of a left lung cyst in Greece 15 years prior to her reoperation here for a lingular recurrence. We conclude that thoracic hydatid disease is a rare entity in northeastern North America. The prevalence of ruptured cysts and infected hepatic cysts involving the lung was higher than in most other series, thereby necessitating more aggressive surgical treatment. Nevertheless, morbidity was low, and recurrence of disease was uncommon in long-term follow-up.