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The nuclei of the olivary complex are involved in regulating motor movements and in motor learning. Aging is characterized by a progressive accumulation of lipofuscin while the number of olivary neurons is unchanged. It is not known whether neuronal loss occurs in the olivary nucleus during Alzheimer's disease (AD). The aim of this study was to estimate the total number of neurons in the principal olivary nucleus (PO) in AD and control brains using an optimized design for sampling and an unbiased stereological method called the optical fractionator. This study was based on 13 control (mean age =57 y, range 17--95 y) and 16 AD brains (mean age = 86 y, range 67--93 y). Using paraffin-embedded, Nissl-stained sections for quantification, we found a significant difference between the mean values (mean \pm SD) of the AD group {\it vs.} aged-matched controls (4.3\times 10^5\pm 1.1 vs. 6.53\times 10^5\pm 1.39 p<0,003). There was no significant difference between younger controls {\it vs.} older controls (7.1\times 10^5\pm 1.5vs. 6.5 \times 10^5\pm 1.4 p=0.76). While there was no neuronal loss in normal aging, in AD the PO is characterized by a significant neuronal loss up to 34%. can occur in regions devoid of neurofibrillary changes. [ABSTRACT FROM AUTHOR]