A case of spontaneous true aneurysm of the left superficial temporal artery (STA) is reported. The patient, a 15-year-old male, had no known history of trauma but had noticed a small, pulsating mass in the left preauricular region 1 month before admission. Physical examination revealed no abnormality other than the 15×10 mm pulsatile mass. Laboratory tests and other routine studies were within normal limits. Left selective external carotid angiography demonstrated a saccular aneurysm of the main trunk of the STA. The aneurysm was resected, and microscopic examination revealed it to be a true aneurysm. Its wall was composed of thickened, uniform tissue with proliferative myointimal cells and pooled acid mucopolysaccharides in the intracellular spaces. Mucoid pools within the residual media separated the cellular elements. In addition, the elastic laminae were disrupted. These findings are quite similar to the characteristic vascular changes of cystic medial necrosis, which often lead to the development of dissecting or saccular aneurysms in large vessels. Aneurysms of the STA are rare and usually post-traumatic. A survey of 165 reported cases of STA aneurysms yielded only 11 cases of spontaneous aneurysms. These aneurysms are assumed to have been congenital or arteriosclerotic. The STA aneurysm described here appears unique, as it was associated with pathological changes resembling those of cystic medial necrosis.