Performing dual tasks, e.g. cognitive and motor tasks simultaneously, may be especially challenging to children with motor disorders. Changes in postural sway have been used to evaluate the effect of dual tasks. Increases in postural sway frequency and concurrent decrease in sway amplitude have been interpreted as tighter control of postural sway. The purpose of this study was to analyze postural sway under single and dual task conditions, while sitting. Thirteen children and adolescents with myelomeningocele and a matched control group were included in the study. The participants performed two single and two dual tasks each. The single task was a sitting still task. The dual tasks were one visual-spatial task and one executive task while simultaneously sitting still. Amplitude, velocity and frequency of center of pressure displacement of postural sway were analyzed between tasks and between groups. The results of the cognitive tasks were analyzed as well. The results revealed different patterns in the groups. During single tasks, the MMC group displayed significantly lower frequencies and velocities of center of pressure displacement compared to the control group. Adding the visual-spatial task influenced postural sway significantly in the control group, while adding the executive task influence the postural sway significantly in the MMC group. The myelomeningocele group confirmed our clinical experience by performing the cognitive tasks slowly but accurately. Further studies are needed to evaluate whether motor function, sensory function, structural anomalies in the brain or any other causes, separately or together, may explain the observed differences in the groups.