There have been few case reports of mandibular dislocation in Parkinson's disease (PD) so far. We experienced habitual mandibular dislocation in two PD patients. They were a 74-year-old woman and a 56-year-old man, who were both graded V by Hoehn and Yahr's staging and had a 13- and 14-year history of PD, respectively. Remarkable retroflexion and rigidity of the neck was observed in both patients. Anterior dislocation of the jaw occurred without any special cause. Once the jaw was dislocated, it required unexpected power to reduce the dislocation against the rigidity. In a case of postencephalitic parkinsonism, continuous parkinsonian tremors of the mandible over a long period was indicated as a cause of the recurrent dislocation seen in the patient. No mandibular tremor, however, was observed in the present cases. The common feature of our two patients was retroflexion of the neck, which was assumed to make the suprahyoid muscles stretched and the mouth opened widely, causing the recurrent mandibular dislocation. Since the recurrence was prevented by chin cap, early chin cap therapy for the mandibular dislocation in parkinsonian patients is recommended.