QTc intervals were measured in 30 patients with Parkinson's disease and 30 healthy control subjects. The mean value of QTc intervals in patients with Parkinson's disease significantly exceeded that of healthy controls (418 +/- 14 ms vs. 403 +/- 18 ms). Prolongation of the QTc interval was unrelated to the administration of levodopa. The mean QTc interval in patients whose Hoehn and Yahr score was III or more was significantly greater than that of patients whose score was II or less (424 +/- 12 ms vs. 410 +/- 12 ms). A significant correlation between the QTc interval and the Valsalva ratio, as well as the overshoot, was identified. The QTc interval is closely related to autonomic nervous system dysfunction, including abnormal baroreceptor reflex function, in patients with Parkinson's disease.