Postmortem counts of dopaminergic cell densities in the substantia nigra (5 subjects) and striatal levels of dopamine (DA) and its metabolites (6 subjects) were determined on 1 parkinsonian (PD), 3 progressive supranuclear palsy (PSP), 1 amyotrophic lateral sclerosis, and 1 Alzheimer's case who had been positron emission tomography scanned with 6-[18F]fluorodopa during life. [18F]Fluorodopa uptake rate constants, which presumably depend on the number of functioning striatal DA terminals, were strictly proportional to cell densities (significant correlation with zero intercept) and also correlated significantly with striatal DA levels but with an intercept indicating greater losses of DA than of terminals in PSP and PD. Postmortem data on 6 PD, 1 PSP, and 9 neuronally normal controls substantiated the significant correlation between cell counts and DA levels, with the latter being the more depressed in pathological cases.