This is the first study to use the unbiased stereological method, the disector, to estimate the total number of pigmented neurons in the pars compacta of the substantia nigra (SNpc) in Alzheimer's disease (AD) patients as compared to healthy controls. The right half of the SNpc of 11 AD patients and 24 controls was studied. We also used single sections to determine the neuronal number and area in different subregions of the SNpc. The results showed that there was no significant difference in the total number of pigmented neurons in the SNpc (154,415+/-13,593 for AD and 160,163+/-8027 for controls) or in the volume of the SNpc between the patients with AD and controls. Studies on single sections revealed that even subregionally there was no significant difference in the neuronal number or area in the SNpc between AD patients and controls.