Relatives of patients with neural tube defects (NTD) frequently request prenatal diagnosis for this problem, and it would be helpful to have information on cost-benefit aspects of offering this service to them. A cost-benefit analysis was done for selective prenatal diagnosis offered to parents and aunts of index cases as these are the two groups for which adequate empiric data are available. The results are extrapolated to other relatives. The study concludes that prenatal diagnosis is cost-beneficial if selectively offered for pregnancies where the fetus is a first or second degree relative of an index case. The results are discussed in terms of whether prenatal diagnosis should be offered to all aunts of index cases or only to maternal aunts. The lifetime costs of caring for an individual with spina bifida, in 1980 dollars, was calculated to be approximately $84,000.