We have investigated regional changes in dopamine metabolism within the basal ganglia with clinical progression of idiopathic Parkinson's disease, using coregistration of [18F]dopa-PET and MRI images and comparing six normal subjects with 15 Parkinson's disease patients in a cross-sectional study. We have demonstrated that [18F]dopa metabolism in the dorsal putamen is reduced to almost 50% of normal both caudally and rostrally early in the disease whilst the ventral putamen is not significantly affected. With progression of symptoms there is loss of dopa metabolism from the ventral putamen, the ventrocaudal putamen in advance of the ventrorostral putamen. Throughout the disease the ventrorostral putamen is relatively preserved; even in the most advanced group [18F]dopa uptake is reduced here by only 30%. We conclude that the progression of Parkinson's disease is associated with a focal process affecting only the dorsal putamen in its early (and preclinical) phase then affecting the ventral putamen with increasing disease severity.