The dopaminergic hypothesis of schizophrenia postulates increased brain dopaminergic activity. Two previous studies reported increased 18F-DOPA uptake with positron emission tomography in schizophrenic patients (n = 5, n = 7). In the present study, striatal dopaminergic function was assessed in vivo in six untreated schizophrenics and seven control subjects, comparable for age and sex. The 18F-fluoro-L-DOPA (18F-DOPA) uptake rate constant Ki was determined in the caudate and putamen using coregistered positron emission tomography and magnetic resonance imaging. No difference between groups for mean Ki was found. The variability of the 18F-DOPA uptake values was higher in the caudate (p < 0.01) and in the putamen (p < 0.001) in schizophrenic patients than in control subjects, suggesting that schizophrenia is a disorder involving heterogeneous states of the striatal presynaptic dopaminergic function.