The syndrome of dopa-responsive dystonia comprises a minority of patients with dystonia, yet it is of considerable diagnostic importance because patients respond dramatically to L-dopa therapy. Benefits from this treatment are lasting, and the problems associated with long-term L-dopa therapy in patients with Parkinson's disease are generally absent. It has been suggested that this condition is due to a defect in the dopamine synthetic pathway, which is bypassed when patients are treated with L-dopa. We have studied [18F]dopa uptake in 6 patients with classic dopa-responsive dystonia (5 familial patients and 1 sporadic patient), aged 18 to 66 years. Data have been analyzed according to a graphic approach, calculating an influx constant for each region studied. We have also studied a seventh, clinically atypical, patient with juvenile dystonia-parkinsonism. Similar data have been calculated for a group of 10 healthy control subjects and 10 patients with Parkinson's disease. The 6 patients with typical dopa-responsive dystonia had a modest but significant reduction in the uptake of tracer into both caudate and putamen, which indicates a defect in the decarboxylation, vesicular uptake, and storage of [18F]dopa. This argues against the proposition that dopa-responsive dystonia is due to an inherited defect of tyrosine hydroxylase alone. In the atypical patient, however, we found a greater reduction of [18F]dopa uptake into both caudate and putamen, comparable with that in patients with Parkinson's disease.