A healthy, nonacclimatized 56-year-old woman developed mood changes and general weakness followed by vomiting, sensory disturbances, and ultimately unconsciousness within hours during an ascent from 1,600 to 2,800 meters in the Himalayas, Nepal. She reported no headache, ataxia, or visual disturbances during and following the hike, as confirmed by fellow travelers. As high-altitude cerebral edema (HACE) was suspected, she received 8 mg of dexamethasone and was transferred to a hospital specializing in acute mountain sickness (AMS) located at 1,300 meters. During the transfer, she had a generalized seizure. The next morning, her consciousness was still clouded. She exhibited subtle, brief, involuntary muscle twitching in both arms and neck. Because she responded properly to stimuli, this was interpreted as myoclonus. Laboratory testing revealed serum hyponatremia (117 mmol/L), hyposmolarity, and urine hyperosmolality. These disturbances were associated with decreased urine volume, high positive fluid balance, anadipsia, and weight gain. Her history revealed multiple prior vaccinations but no infections.