The aetiology of gait disturbances in Parkinson's disease (PD) is not fully understood. Recently, it was shown that in patients with PD, bilateral coordination of gait is impaired and that walking while being simultaneously engaged in a cognitive task is detrimental to their gait. To assess whether cognitive function influences the bilateral coordination of gait in PD, this study quantified left-right stepping coordination using a phase coordination index (PCI) that evaluates both the variability and inaccuracy of the left-right stepping phase (φ) generation (where the ideal φ value between left and right stepping is 180°). This report calculated PCI values from data obtained from force sensitive insoles embedded in subjects' shoes during 2 min of walking in a group of patients with PD (n = 21) and in an age matched control group (n = 13). All subjects walked under two walking conditions: usual walking and dual tasking (DT) (ie, cognitive loading) condition. For patients with PD, PCI values were significantly higher (ie, poorer coordination) during the DT walking condition compared with usual walking (p<0.001). In contrast, DT did not significantly affect the PCI of the healthy controls (p = 0.29). PCI changes caused by DT were significantly correlated with changes in gait variability but not with changes in gait asymmetry that resulted from the DT condition. These changes were also associated with performance on a test of executive function. The present findings suggest that in patients with PD, cognitive resources are used in order to maintain consistent and accurate alternations in left-right stepping.