A 46-year-old woman presented with malignant ophthalmopathy 1 week after a therapeutic dose of radioiodine for treatment of hyperthyroidism. The patient was a smoker and had clinical evidence of mild thyroid-associated ophthalmopathy (TAO) prior to treatment with radioiodine. Anti-thyrotropin (TSH) receptor antibodies and antiflavoprotein antibodies were not detected at the time of presentation with malignant ophthalmopathy. The patient responded rapidly to anti-inflammatory treatment with intravenous methylprednisolone and orbital radiation.