To report a case of bilateral Marcus Gunn syndrome, which can be easily overlooked.

A 6-year-old boy visited our clinic complaining of right ptosis. On ocular examination, the palpebral fissure width was 4 mm in the right lid and 7 mm in the left lid. Marginal reflex distance 1 (MRD 1) was -1 mm in the right lid and 2 mm in the left lid. Bilateral levator muscle function was good. When the patient moved the jaw to the right side, the left eyelid was retracted by 2 mm, and when the jaw was moved to the left side, the right upper eyelid retracted by 4 mm, which showed a moderate bilateral jaw-winking phenomenon. There was no change in bilateral palpebral fissure width when only opening the mouth without jaw movement.

Asymmetrical bilateral ptosis can be easily misdiagnosed for unilateral ptosis, and concurrent Marcus Gunn phenomenon can exist. Therefore, a thorough examination is necessary when examining a ptosis patient.