Amniocentesis and chorionic villus sampling are tests for the prenatal diagnosis of cytogenetic abnormalities. We calculated the incremental costs per abnormal birth averted, and the incremental costs per quality-adjusted outcome, of amniocentesis and chorionic villus sampling performed for the indication of maternal age. Probabilities were obtained from the literature, and direct medical costs from hospital charges deflated to reflect aggregated contracted care reimbursements. Utilities were used to quality-adjust prenatal testing outcomes. Based on costs per abnormal birth averted, at all maternal ages from 30 to 43 years, amniocentesis was more cost-effective than chorionic villus sampling; at ages 44 and 45, chorionic villus sampling was more cost-effective. However, if the anxiety reduction provided by first-trimester diagnosis was equivalent to a 0.2% risk of an abnormal child, chorionic villus sampling was more cost-effective than amniocentesis at all maternal ages. Based on data from the 1988 United States natality cohort, the current policy of testing women aged 35 and older would cost $103,329 and $111,184 per abnormal birth averted for amniocentesis and chorionic villus sampling, respectively. Testing women aged 30 and older would almost double these costs. For either prenatal test, targeting high risk women for testing, and striving for utilization rates of 50% or higher, appeared to be the most cost-effective policy.