Patients with Parkinson's disease (PD) often experience freezing of gait, a debilitating phenomenon during which the subject suddenly becomes unable to start walking or to continue to move forward. Little is known about the gait of those subjects with PD who experience freezing of gait or the pathophysiology of freezing. One possibility is that freezing of gait is a truly paroxysmal phenomenon and that the usual walking pattern of subjects who experience freezing of gait is not different than that of other patients with PD who do not experience these transient episodes of freezing of gait. On the other hand, a recent study noted gait changes just prior to freezing and concluded that dyscontrol of the cadence of walking contributes to freezing. To address this question, we compared the gait of PD subjects with freezing of gait to PD subjects without freezing of gait. Given the potential importance of the dyscontrol of the cadence of walking in freezing, we focused on two aspects of gait dynamics: the average stride time (the inverse of cadence, a measure of the walking pace or rate) and the variability of the stride time (a measure of "dyscontrol," arrhythmicity and unsteadiness). We found that although the average stride time was similar in subjects with and without freezing, stride-to-stride variability was markedly increased among PD subjects with freezing of gait compared to those without freezing of gait, both while "on" (P<0.020) and "off" (P<0.002) anti-parkinsonian medications. Further, we found that increased gait variability was not related to other measures of motor control (while off medications) and levodopa apparently reduced gait variability, both in subjects with and without freezing. These results suggest that a paradigm shift should take place in our view of freezing of gait. PD subjects with freezing of gait have a continuous gait disturbance: the ability to regulate the stride-to-stride variations in gait timing and maintain a stable walking rhythm is markedly impaired in subjects with freezing of gait. In addition, these findings suggest that the inability to control cadence might play an important role in this debilitating phenomenon and highlight the key role of dopamine-mediated pathways in the stride-to-stride regulation of walking.