An autopsy case of spontaneous rupture of a clinically undiagnosed inflammatory aneurysm of the aortic sinus is presented. A 34-year-old man consulted a physician complaining of anterior chest pain and heartburn persisting for 2 days. Diagnoses of common cold and gastritis were made, but 3h later his condition deteriorated, leading to death. Autopsy revealed hemopericardium due to rupture of a saccular aneurysm of the aortic sinus. Histological examination of the aortic sinus disclosed aortitis characterized by fragmentation and loss of elastic tissues with fibrous thickening of the intima. Lymphocytes along with polymorphonuclear leucocytes and plasma cells infiltrated the adventitia and media at severely affected areas. These changes were limited to the aortic sinus, and other portions of the aorta and its main branches were essentially intact. An eccentric intimal thickening with medial degeneration was found within the anterior descending branch of the left coronary artery, and occasional collection of lymphocytes was noted in the epicardium. This case would be a rare and illustrative case for differentiation of aortitis, although we could not pinpoint the etiology of the aortitis because of non-specific histological findings and negative serological results for exogenous pathogens.