Symptoms of abnormal autonomic-nervous-system function occur commonly in Parkinson's disease (PD). Orthostatic hypotension in patients with parkinsonism has been thought to be a side-effect of treatment with levodopa, a late stage in the disease progression, or, if prominent and early with respect to disordered movement, an indication of a different disease, such as multiple system atrophy. Instead, patients with PD and orthostatic hypotension have clear evidence for baroreflex failure and loss of sympathetic innervation, most noticeably in the heart. By contrast, patients with multiple system atrophy, which is difficult to distinguish clinically from PD, have intact cardiac sympathetic innervation. Postmortem studies confirm this distinction. Because PD involves postganglionic sympathetic noradrenergic lesions, the disease seems to be not only a movement disorder with dopamine loss in the nigrostriatal system of the brain, but also a dysautonomia, with norepinephrine loss in the sympathetic nervous system of the heart.