Symptoms and signs of postural instability are often present in patients with Parkinson disease (PD). Because vestibular dysfunction might contribute to the postural instability, we carried out bithermal caloric tests and electronystagmography in 36 patients with PD and 316 controls. Significantly more PD patients had reduced and absent vestibular responses than controls. Decreased or absent vestibular responses in patients with PD were associated with postural instability and increasing severity of other symptoms. Vestibular dysfunction may contribute to the postural instability of PD.