A 48-year-old fisherman experienced the sudden onset of a headache during a skin-diving for fish in the sea. A computed tomography revealed a subarachnoid hemorrhage (SAH). Cerebral angiogram showed an aneurysm in the bifurcation of the right middle cerebral artery. The next day, neck clipping of the aneurysm was performed. The clipping was completed, and 1 month later, he was discharged without neurologic deficits. He had been treated with an antiplatelet agent for a previous right medullary infarction for 1 year. At the start of treatment for the medullary infarction, a magnetic resonance angiogram showed atherosclerotic changes with a small aneurysm in the right middle cerebral artery bifurcation. A first year follow-up magnetic resonance angiogram after the infarction revealed the aneurysm had increased substantially in size. Repeated pressure changes owing to his repeated diving might have triggered the aneurysmal ballooning and rupture.