Striatal dopamine receptors were studied in 44 patients with Parkinson disease by the radioligand-binding technique using /sup 3/H-spiroperidol. The specific binding of /sup 3/H-spiroperidol was either significantly increased or reduced in the caudate nucleus and putamen of parkinsonian patients without levodopa therapy. Scatchard analysis showed that there were corresponding changes in the receptor number, but no significant changes in the mean dissociation constant. The increased binding of /sup 3/H-spiroperidol in the basal ganglia was also found in parkinsonian patients suffering from psychotic episodes and treated with neuroleptic drugs. Normal and low binding of /sup 3/H-spiroperidol was found in patients treated with levodopa. Clinically, the patient with low binding were more disabled and had lost the beneficial response to levodopa. Thus in Parkinson disease in some patients a denervation supersensitivity seemed to develop and in some others a loss of postsynaptic dopamine receptor sites in the neostriatium. The latter alteration may contribute to the decreased response of parkinsonian patients to chronic levodopa therapy.