In line with the view that aneuploidy destabilizes the karyotype, initiating an autocatalytic process that gives rise to further loss and/or gain of chromosomes, we examined whether a constitutional aneuploidy such as monosomy for one chromosome is associated with sporadic loss and/or gain of other chromosomes. We used PHA-stimulated lymphocytes from eight women with Turner's syndrome (six displayed X chromosome monosomy ranging from 60.2% to 97.9%, and two were below 10%), and eight healthy women who served as a control group. Fluorescence in-situ hybridization (FISH), applied at interphase, was used to evaluate the level of aneuploidy for three randomly selected chromosomes (autosomes 8, 15 and 18) in each sample. For each tested chromosome, our results showed a significantly higher level of aneuploid cells in the samples from patients than in those from controls (p < 0.01). The mean level of aneuploid cells for all three tested autosomes was almost twice as high in the patient samples as in the control samples (p < 0.002). It is noteworthy that, in the Turner's syndrome patients, X chromosome disomic cells also displayed increased levels of aneuploidy. It is possible that monosomy of X chromosome in female cells destabilizes their own genome and also affects X disomic cells in the region. One may also speculate that a common factor(s) is involved with both constitutional and sporadic aneuploidy.