Fluorodopa (18F-DOPA) is used with positron emission tomography (PET) to visualize the regional distribution of the neurotransmitter dopamine in the human brain. 18F-DOPA with PET is used to assess presynaptic dopaminergic function in vivo in the human brain and to examine changes in dopamine neurotransmission in clinically dysfunctional patients. Since patients with Parkinson's disease have been demonstrated to have low 18F-dopa uptake throughout the striatum, 18F-DOPA with PET may be used to confirm the diagnosis of Parkinson's disease and to follow the rate of disease progression. A decreased 18F-DOPA uptake, especially in the putamen, may be indicative of a primary degeneration of the dopaminergic nigrostriatal neurons typical for idiopathic Parkinson's disease. Also, 18F-DOPA has been used to investigate the effect of medications in Parkinson's disease, particularly of catechol O-methyltransferase (COMT) inhibitors (e.g., nitecapone and entacapone), on the accumulation of dopamine in the striatum. 18F-DOPA may be useful for identifying preclinical or early Parkinson's disease in clinically normal subjects. 18F-DOPA with PET may be helpful in confirming clinical suspicions in the rare cases of psychogenic parkinsonism.