The possibility that female carriers of the fragile X gene(s) are at increased risk for nondisjunctional events leading to aneuploid offspring has been suggested by several investigators. To better address this question we analyzed pedigrees of 117 families in which the fragile X syndrome is segregating. The 117 pedigrees, originally collected for segregation analyses, included 236 females with offspring whose carrier status was determined by cytogenetic or pedigree analysis or by analyses using flanking DNA markers. These 236 females have had 931 offspring including one 47,XXY and 6 trisomy 21 individuals (1/155). Statistical analysis suggested that the observed rate of trisomy 21 was significantly higher than expected (Fisher's exact test, p less than or equal to 0.05). Assuming a Poisson distribution to calculate the confidence interval for the observed rate of trisomy 21 individuals, we found that the expected rate of 1.6/1000 in this sample fell outside the 99% confidence limits of our observed rate of 1/155. Additional data from a larger sample are needed to replicate these findings.