Background: The authors identified the second known mutation in the α-synuclein (SNCA) gene, an alanine-to-proline exchange in amino acid position 30 (A30P), that cosegregates with the disease in one German family with autosomal dominantly inherited parkinsonism (ADP). The authors studied carriers of the A30P mutation to compare the phenotype of this mutation with idiopathic PD (IPD) and to assess nigrostriatal dopaminergic function in symptomatic and preclinical mutation carriers. Methods: The pedigree of the A30P family spans five generations with five affected individuals. The authors performed detailed neurologic examinations followed by mutation analysis in 11 living individuals. In three mutation carriers, two individuals with definite PD and one person at risk for PD, they used L- F-fluoro-3,4-dihydroxyphenylalanine (F-DOPA), C-raclopride (RAC), and F-fluorodeoxyglucose (FDG) PET to investigate presynaptic dopaminergic function, dopamine D2 receptors, and cerebral energy metabolism. The authors studied the cognitive functions of carriers of the A30P mutation using neuropsychological screening. Results: PET studies revealed striatal presynaptic dopaminergic alterations consistent with sporadic IPD in two affected family members and no evidence for nigrostriatal dopaminergic dysfunction in one presymptomatic mutation carrier. Neuropsychological testing in four mutation carriers provided evidence for cognitive impairment as a frequent and early symptom of the A30P mutation; this is also supported by regional cerebral energy metabolism alterations in the clinically presymptomatic subject. Conclusions: The phenotype of the A30P mutation in the SNCA gene is similar to that of sporadic IPD, including a high variability of the age at disease onset, ranging from 54 to 76 years. The follow-up of presymptomatic carriers of the A30P mutation may give insight into preclinical disease stages and early manifestations of PD.