A 68-year-old man presented with unilateral ptosis and no other symptoms. The neurologic examination revealed ptosis of the left eye after a sustained upward gaze (Panel A). The movements of the extraocular muscles were normal. Myasthenia gravis was suspected, and the ice-pack test was performed with the placement of an instant cold pack over the left eye (Panel B). After 2 minutes, the ptosis was substantially diminished (>5 mm), indicating a positive test (Panel C). The diagnosis was further supported by the presence of serum anti–acetylcholine receptor antibodies and by electrodiagnostic testing, which showed a decremental response to repetitive nerve stimulation. The ice-pack test can be a useful bedside test to distinguish myasthenia gravis from other causes of ptosis or ophthalmoparesis. The inhibition of acetylcholinesterase activity at a reduced muscle temperature is thought to underlie the observed clinical improvement. The patient was treated symptomatically with pyridostigmine, and the ptosis was diminished.