Internuclear ophthalmoplegia in the Chiari type II malformation. We describe 3 cases of Chiari type II malformation presenting with bilateral internuclear ophthalmoplegia (INO). Although prominent, the INO was not an isolated sign in any of the patients; superimposed abduction paresis was present in 2, and deficits in smooth pursuit, optokinetic nystagmus, and vestibulo-ocular responses were present in 3. Two had hydrocephalus: 1 was clinically unchanged without therapy after 5 years; the other did not improve with shunting. Findings in our 3 patients, along with the 4 previously reported, confirm that INO is 1 manifestation of widespread brainstem or cerebellar dysfunction. Its origin is probably multifactorial, related to hydrocephalus, vascular compromise, direct neuronal distortion, or congenital neural malformation. 