Premature centromere division (PCD) of the chromosome 18 was analyzed by using fluorescent in situ hybridization (FISH) on interphase peripheral blood lymphocytes isolated from six sporadic Alzheimer disease (AD) patients and six healthy elderly controls. Results of FISH analysis revealed that chromosome 18 expressed PCD in 5.18% interphase nuclei of AD patients, and in 2.59% interphase nuclei of age-matched controls (p<0.05). Our study also showed that hypoploidy and hyperploidy frequency for chromosome 18 exhibited a statistically significant increase in the AD group compared to the control one. The increase in spontaneous aneuploidy of chromosome 18 in AD patients which is correlated with PCD shows that deregulation of the time of centromere separation can be considered as a manifestation of chromosome instability leading to aneuploidy.