This report concerns 30 patients with the acute form of "inflammatory aneurysm" of the aorta treated during a 27-year period between April 1957 and March 1984. There were 28 men and two women whose ages ranged from 46 to 78 years (most over 60 years). All were heavy smokers. The aneurysmal disease was located below the renal arteries and the inflammatory changes were limited to the abdomen in 24 patients; one patient had a ruptured aneurysm. The aneurysm involved the entire abdominal aorta in one, the descending thoracic and infrarenal abdominal aorta in two, and the descending thoracic and abdominal aorta in continuity in three patients. The inflammatory changes occurred grossly and microscopically in both abdominal and thoracic aortic segments in five of the latter six patients. The changes were manifested by anterior and lateral mural inflammatory thickening contiguous with similar changes of the retroperitoneum and mediastinum that produced varying degrees of ureteral obstruction in seven patients. Most had abdominal, back, or flank pain and abdominal tenderness, suggesting rupture or leakage. Emergency exploratory operation had been performed elsewhere in 10 patients. Operation was abandoned because of exposure difficulties, bowel perforation, or visceral arterial involvement. Diagnosis was suggested by CT scan in 10, ultrasonography in one, and excretory urograms in seven patients. Treatment consisted of thoracoabdominal aortic replacement in six and infrarenal aortic replacement in 24. The aorta was clamped at the diaphragm in most of the latter cases to avoid injury to adjacent structures. Nephrectomy or ureterolysis was rarely necessary. Of these 30 patients, 29 were early (30 day) survivors and ureteral obstruction spontaneously subsided in most cases without special treatment. There was one late death at 2 months and eight deaths from 3 to 13 years; 20 (67%) patients are still alive.