A nondissecting aneurysm of the distal descending thoracic aorta, 55 mm in diameter, in a 35-year-old woman who was undergoing 24-year steroid therapy of systemic lupus erythematosus was successfully resected and replaced with a tube graft. The suture line of the aortic wall was reinforced with a polytetrafluoroethylene felt strip. Histologically, the media and adventitia contained numerous lymphoplasmacytic infiltrates that consisted of B and activated T cells mixed with plasmacytes. These findings would have been in accordance with active aortitis. Systemic lupus erythematosus was suggested as possibly associated with nondissecting thoracic aortic aneurysm, especially in patients undergoing chronic steroid therapy in which inflammation continued.