PURPOSE: Down syndrome (DS) is associated with reduced visual performance. Although poor optical quality has been implicated, no previous data are available regarding the contribution of cortical visual processes. The present study investigated Vernier performance for the first time in children with DS to evaluate the integrity of higher visual processing in this condition. METHODS: Participants were 29 children aged 9 to 16 years who had DS and 68 age-matched developmentally normal children acting as controls. All wore best refractive correction, and none had clinically significant ocular abnormalities. An out-of-phase test-pedestal Vernier stimulus was used to facilitate short test distances and optimize compliance with testing. RESULTS: Testing was successfully completed by 86% (n=25) of the DS group and 96% (n=65) of the control group. Vernier thresholds were invariant with age in both groups. Mean Vernier acuities were 39.8 arc seconds (SD+/-13.3) and 14.6 arc seconds (SD+/-4.7) in DS and control groups, respectively. When compared with control data, mean Vernier acuity was reduced by a factor of 2.7 in DS. CONCLUSIONS: Vernier thresholds were successfully measured in children with DS and were found to be reduced, indicating that cortical visual function is compromised. Impairment in cortical function in DS may be implicit, relating to histologic reports of differences in the DS brain, or they may result from abnormal experience during visual development. The magnitude of the cortical deficit demonstrated in DS in the present study is significant and should be considered along with previously reported poor optical quality.