Freezing of gait (FOG) is a common and disabling feature of Parkinson's disease (PD). Detailed pathophysiological studies are hampered by the fact that FOG episodes are difficult to elicit in a gait laboratory. We evaluated whether the need to avoid sudden obstacles on a treadmill can provoke FOG. We included 21 PD patients (15 with self-reported off-period FOG). Patients were tested in the off-state. FOG during overground walking was assessed using a standardized gait trajectory and axial 360 degrees turns. Subsequently, patients walked on a motorized treadmill with suddenly appearing obstacles that necessitated compensatory stepping. Performance was videotaped, and presence of FOG was scored visually by two independent raters. Thirteen patients showed FOG during overground walking. During treadmill walking, obstacle avoidance was associated with 13 unequivocal FOG episodes in eight patients, whereas only one patient froze during undisturbed treadmill walking (Wilcoxon z = -2.0, P = 0.046). FOG episodes elicited by obstacle avoidance were brief (typically <1 s). Almost all episodes were provoked when subjects had a longer available response time. In conclusion, suddenly appearing obstacles on a treadmill can elicit FOG in a controlled laboratory setting. However, the moving treadmill and the obstacle both act as cues, which apparently help to immediately overcome the provoked FOG episode. This may limit the ecological validity of this new approach.