The local concentration of 6-[18F]fluoro-L-dopa (18F) reflects the activity of aromatic acid decarboxylase (AADC), the enzyme that generates dopamine from its precursor amino acid, L-dopa. In young healthy adults, the local concentration of 18F, and hence AADC activity, is constant in coronal slices taken in a rostrocaudal direction. With increasing age a gradient representing decreasing activity in the putamen develops. This decrease is less marked than was expected from the literature. In five children with primary dystonia, the striatal distribution of 18F resembled that seen in the normal older adults. In established clinical Parkinson's disease the rostrocaudal gradient becomes steep; the putamen is more damaged.