Of 11 young children (six girls and five boys, ranging in age from 4 to 14 months) with monocular nystagmus, six were found to have chiasmal tumors whereas only four had spasmus nutans. Differentiating features between these two groups included decreased vision and optic nerve anomalies (including four cases of optic nerve hypoplasia) in the tumor group. However, we found that no consistent clinical finding absolutely separates these groups. We therefore recommend that all infants and children with monocular nystagmus or the other features of spasmus nutans should undergo computed tomographic scanning.