Many patients with Alzheimer's disease (AD) develop parkinsonian symptoms, suggesting an overlapping between AD and Parkinson's disease (PD). However, pathologic and neurochemical studies indicate that the involvement of the dopamine system may be different in the two conditions. Using single photon emission tomography, we determined the relative specific striatal uptake (striatum to cerebellum ratio) of the D2 receptor ligand [123I]-IBZM in 15 AD patients without overt extrapyramidal symptoms (three subjects presented mild rigidity and bradykinesia) and nine age-matched controls. Mean specific activity in striatal regions of AD patients (1.35 +/- 0.09) was significantly reduced from control mean (1.59 +/- 0.03). Because such changes were evident even in the absence of overt parkinsonian symptomatology, our data indicate that alterations of striatal D2 receptors may be part of the pathologic abnormalities of AD. In addition, the mechanisms underlying extrapyramidal symptoms in AD (decline of postsynaptic striatal dopamine receptors) appear different from the prevalent presynaptic nigrostriatal alterations typical of PD.