Internuclear ophthalmoplegia (INO) is a sign of exquisite localizing value, often due to either multiple sclerosis or infarction. To demonstrate that unusual causes of INO are more common than the 11% reported in previous series, this review considers a case series of 410 inpatients whom I personally examined during a 33-year period. In this series, the cause of INO was infarction in 157 patien...