The amniotic fluid alpha-fetoprotein assay (AF-AFP) is the single most valuable method of detecting anencephaly--spina bifida (ASB) in utero. A raised AF-AFP, however, is a more reliable indicator of a fetus with ASB in the pregnancy at high risk for ASB. Most women undergoing fetal chromosomal analysis are at low risk for ASB. Consequently, gravid patients who have an AF-AFP incidental to chromosomal studies, and elect abortion because of a raised AF-AFP, will more frequently abort a normal fetus than the gravida who has previously had an infant with ASB. For the obstetrician utilizing this assay, the predictive value positive (PVP) concept, a guide for the interpretation of a positive result, is introduced and then applied to examples of patients at varying risk for ASB in their offspring.