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 patients (38%), multiple sclerosis in 139 (34%), and unusual causes in 114 (28%). Unusual causes included trauma (20 cases), tentorial herniation (20 cases), infection (17 cases), tumor (17 cases), iatrogenic injury (12 cases), hemorrhage (13 cases), vasculitis (7 cases), and miscellaneous (8 cases). Internuclear ophthalmoplegia was unilateral in 136 of the infarct cases (87%), 38 of those with multiple sclerosis (27%), and 48 of the unusual cases (42%). Because unusual causes compose more than one quarter of the cases, the differential diagnosis of INO should be tripartite: multiple sclerosis, stroke, and other causes.