Myasthenia gravis is the most common disorder of neuromuscular transmission and is a heterogeneous disorder that is generally autoimmune, which is caused by an auto-antibody to the nicotinic acetylcholine receptor. We present a rare case of myasthenia gravis that occurred 8 weeks after a coronary artery bypass grafting operation. A 64-year-old man with multivessel coronary artery disease underwent myocardial revascularization. Severe myasthenic symptoms began 8 weeks after the operation and emergent mechanical ventilation was needed because of myasthenic crises. The serum anti-acetylcholine receptor antibody level was high. The details of this unusual patient are discussed.