METHODS: We measured blood-brain barrier transport and decarboxylation of 6-[18F]fluoro-L-DOPA (FDOPA) using PET in patients with Parkinson's disease (n = 7, 57 +/- 7 yr) and age-matched control subjects (n = 7, 60 +/- 6 yr). To visually present regional changes of FDOPA uptake in Parkinson's disease, we introduced maps of FDOPA uptake relative to occipital cortex, averaged across control subjects and Parkinson's disease patients in an MRI-based stereotaxic coordinate space. RESULTS: There was no significant changes in the blood-to-brain transport of FDOPA (KD1) in Parkinson's disease. The KD1 values of the head of caudate were lower than those of putamen in both normal subjects and Parkinson's disease patients. In Parkinson's disease, the activity of L-DOPA decarboxylase (DDC) was differentially reduced in subdivisions of striatum. The residual DDC activity was 63% of the control value in the head of caudate nucleus, 54% in the anterior putamen and 39% in the posterior putamen. The DDC activity in frontal and occipital cortices remained unchanged by the disease. Subtraction of averaged FDOPA uptake maps (control minus Parkinson's disease) visualized a spatial pattern of pathological changes in FDOPA uptake common to Parkinson's disease patients. CONCLUSION: The striatal blood-to-brain transport of FDOPA remained unchanged while the DDC activity was differentially reduced within the striatum in Parkinson's disease. We found the FDOPA uptake maps useful in identifying altered patterns of FDOPA metabolism common in Parkinson's disease.