Advanced maternal age is the only well-established risk factor for trisomy 21 Down syndrome (DS), but the basis of the maternal-age effect is not known. In a population-based, case-control study of DS, women who reported surgical removal of all or part of an ovary or congenital absence of one ovary were significantly more likely to have delivered a child with DS than were women who did not report a reduced ovarian complement (odds ratio 9.61; 95% confidence interval 1.18-446.3). Because others have observed that women who have had an ovary removed exhibit elevated levels of FSH and similar hallmarks of advanced maternal age, our finding suggests that the physiological status of the ovary is key to the maternal-age effect. In addition, it suggests that women with a reduced ovarian complement should be offered prenatal diagnosis.