Objective: Diplopia is not frequently associated with Arnold-Chiari I malformation. We reviewed our cases of Arnold-Chiari I malformation in which acquired esotropia with diplopia was the main neuro-ophthalmologic finding early in the clinical course. Materials and Methods: Five patients were studied, all female, ranging in age from 17 to 36 years, who were treated by the neuro-ophthalmology service of urban teaching hospitals. Eye movement recordings using magnetic search coil technique were performed in two patients. Results: All patients reported onset of horizontal diplopia due to acquired esotropia as an initial manifestation of the Arnold-Chiari I malformation. All had full abduction of each eye. In addition, all five had gaze-evoked nystagmus, two skew deviations, and one bilateral internuclear ophthalmoparesis. Oculography in two patients showed normal abducting saccadic peak velocities. This supports divergence palsy as a mechanism of acquired esotropia and provides evidence against subtle sixth nerve palsy in these patients. Four patients underwent neurosurgical decompression of their Chiari malformations, and neuro-ophthalmologic signs and symptoms improved in all. Conclusions: Acquired esotropia, often in association with other eye movement abnormalities, may be an early sign of Arnold-Chiari 1 malformation. This quantitative study indicates that divergence palsy is the cause of esotropia in some patients. Neurosurgical suboccipital and upper cervical decompression may lead to improvement or resolution of diplopia.