Freezing of gait (FOG) in Parkinson's disease (PD) occurs most frequently during turns or step initiation, two tasks that likely demand a high degree of bilateral coordination between the legs. Our objective was to test the hypothesis that impairments in bilateral coordination of stepping are associated with FOG in PD. We compared locomotion features while walking on level ground between patients with PD that experience FOG (PD + FOG; n = 21) and patients with PD that do not (PD - FOG; n = 13). To study bilateral stepping coordination, we defined the stride duration of one foot as a gait cycle or 360 degrees , determined the relative timing of contralateral heel-strikes and defined this as the phase, phi (ideally, phi = 180 degrees ). The sum of the coefficient of variation of phi and the mean absolute difference between phi and 180 degrees was defined as the phase coordination index (PCI), representing variability and inaccuracy, respectively, in phase generation. During the 'Off' state (= 12 h off anti-parkinsonian medication), PCI values were higher (poorer coordination) in PD + FOG compared with PD - FOG (P < 0.024). Stride-to-stride phase adjustments, Deltaphi, were also studied. Both groups scaled their 'converging' adjustments (towards 180 degrees ) to the same extent, but when generating diverging Deltaphi (away from 180 degrees ), PD + FOG patients exhibited larger errors compared with PD - FOG patients (P < 0.006). This study demonstrates that patients with PD who experience FOG have distinctive impairments in the bilateral coordination of locomotion. Poor bilateral coordination of walking may predispose to FOG, especially during challenging tasks that demand a high degree of left-right coordination.