A 31-year-old man suffered diplopia and ataxia on two occasions when he ascended from sea level to 4,000 m. Evaluation revealed a moderate-sized subarachnoid cyst in the left frontal region, which did not communicate with the cerebral ventricles. The cyst might have acted as a space-occupying lesion, and caused symptoms on ascent due to hypoxic brain swelling, brain compression against the cyst, and elevated intracranial pressure. Subarachnoid cysts are common, and they should be considered in the differential diagnosis of neurological problems at high altitude.