[18F]6-fluoro-L-dopa and positron emission tomography has been used to study intracerebral dopamine distribution in five control subjects and six patients with hemiparkinsonism. In the control subjects striatal, frontal and cingulate accumulations were clearly seen. In addition 18F concentrated in the region of the insula and the parietal lobe. In the patients striatal accumulation 18F was reduced in the contralateral striatum, especially in the putamen. The uniformity of distribution of 18F in the striatum on the side of the parkinsonian signs was also irregular. This finding is consonant with the suggestion that intracerebral compensatory mechanisms prevent the manifestation of intracerebral dopamine deficiency from becoming obvious until a late stage of the disease.