To test the hypothesis that parental aneuploidy, particularly involving sex chromosomes, indicates an increased risk of meiotic nondisjunction, we analyzed chromosome numbers from 283 parents who had had 2 or more spontaneous abortions and, if they had children, no abnormal offspring. A sample of 15 lymphocyte metaphases was examined per individual for a total of 4,245 metaphases. Two or more cells with the same abnormal chromosome complement were found in 19 individuals: 4 males with a minor number of 45,X cells and 15 females with minor numbers of 45,X and/or 47,XXX cells (p less than 0.001). The most logical explanation is that mitosis may, in part, reflect meiosis. This group of parents appears predisposed to chromosome errors in meiosis leading to recurrent spontaneous abortion.