The suggestion has been made that the women who produce gametes trisomic for chromosome 21, might also be liable to produce gametes monosomic for this chromosome. If so, the gamete would probably be inviable, and might result in an abortion. A high rate of abortion has been observed in the mothers of mongols. If some of these abortions are monosomic lethals, they could be expected to appear late rather than early in the sibship. This possibility has been examined by an analysis of the mongol sibships reported by Hanhart. In Hanhart''s material it is found that, independently of age, the incidence of miscarriage tends to run higher than in reported samples of mothers not selected for having borne a mongol. In women of Hanhart''s groups I and II, who bore the mongol propositus at ages less than 37, there was no rise in the incidence of miscarriage with increasing age. In women of group HI, who bore a mongol child at ages from 37 on, there was no rise in incidence of miscarriage with increasing age in earlier life, but there was such a rise in incidence from 35 onwards. This tends to support the possibility that some of these abortions are due to the appearance of monosomic lethals. The opportunity was taken of examining the question of individual susceptibility to repeated miscarriage. In women who had once miscarried, the risk of a later miscarriage was found to be greatly increased. This is not thought to have any bearing on the problem discussed above.