Thoracoabdominal operation was not performed in 94 patients with thoracoabdominal aortic aneurysms during a 25-year period because of refusal by patients (eight patients), associated disease or age (64 patients), small size of aneurysm (five patients), and staging of operations in which another operation was performed instead of aneurysm resection (17 patients). The cause of aneurysm was chronic dissection in 23 patients and nondissection in 71. Most of the patients entered the study during the past 5 years and follow-up information is complete for all of them. Twenty-four percent of the patients were alive at 2 years, calculated by the Kaplan-Meier method, and half of the deaths were due to aneurysm rupture. In contrast, among our 604 patients treated surgically, 70% were alive at 2 years and 59% at 5 years. This study suggests that the natural course of the disease is favorably altered by surgical treatment.