Striatal dopamine D2 receptors were studied, using positron emission tomography (PET), in 10 patients with early Parkinson's disease without any antiparkinsonian medication and in 14 healthy controls. [11C]Raclopride was used as ligand and an equilibrium method was applied. The maximum count of receptors (Bmax) and their dissociation constant (Kd) were calculated according to the Scatchard principle. In parkinsonian patients, the Bmax of D2 receptors was increased in the putamen contralateral to the predominant symptoms, as compared to the opposite putamen, by 33% (p = 0.0008). In the caudate nucleus no significant side to side differences was noted. On comparison with age-matched healthy controls, Bmax values in the putamen (p = 0.0012) but not in the caudate nucleus contralateral to the side of predominant clinical symptoms were increased in PD patients. The Kd values were unchanged. The difference in putaminal Bmax values between the opposite hemispheres correlated with the difference in the severity of parkinsonian motor symptoms between the two body sides (r = 0.69, p = 0.03). The present results show that there is both a relative and absolute increase in the number of dopamine D2 receptors in the putamen, but not in the caudate nucleus in early Parkinson's disease.