Positron emission tomography with 6-[18F]fluoro-L-dopa (6-FD) provides in vivo information on the function of nigrostriatal dopaminergic neurons. We used 6-FD and positron emission tomography to investigate the integrity of the nigrostriatal system in seven patients with progressive supranuclear palsy. All patients had axial hypertonia, vertical gaze palsy, and parkinsonian features. Dementia, pyramidal signs, and ataxia were seen in varying proportions. We analyzed the scans with a graphic method to calculate a steady-state 6-FD uptake rate constant for the whole striatum. Results were compared with those obtained in seven age-matched controls. As a group, the patients with progressive supranuclear palsy had reduced 6-FD uptake constants. The 6-FD uptake constant correlated inversely with the duration of the disease. Normal positron emission tographic findings in one patient with the shortest duration of symptoms suggests that in early progressive supranuclear palsy, parkinsonism may relate to dysfunction distal to the dopaminergic neurons.