We studied abnormalities of eye movement in five patients with Arnold-Chiari malformation before and after suboccipital decompression. Before operation all patients complained of oscillopsia at rest and visual blurring of fixated targets during and immediately after their heads were in motion. Three patients had downbeat nystagmus, one patient had horizontal nystagmus in the primary position and on eccentric gaze and one patient had rotatory nystagmus on lateral gaze only. Smooth pursuit, optokinetic nystagmus and fixation suppression of vestibular nystagmus were markedly impaired. Within two months after operation nystagmus in the primary position had resolved in three patients, and four had complete to near-complete resolution of nystagmus in all gaze positions over subsequent months. Oscillopsia decreased concurrently. affording significant functional visual improvement. One patient had only slight functional improvement and continued to have prominent nystagmus seven years after operation. Smooth pursuit, optokinetic nystagmus and fixation suppression of vestibular nystagmus improved in all patients, but none had complete recovery at the higher velocities of stimulation. They still complained of visual blurring associated with head motion, probably due to the poor fixation suppression of vestibular nystagmus. Our findings suggest that abnormalities of eye movement in Arnold-Chiari malformation are due to compression of the herniating cerebellum against the caudal brain-stem and not a congenital aberrancy of oculomotor pathways. Nystagmus resolved more readily than pursuit or fixation suppression abnormalities, but most patients showed gradual improvement in all abnormalities four months after operation.