The prediction of neonatal alloimmune thrombocytopenia (NATP) in affected families has, in the past, been based on information about gene frequencies of the antigen systems involved, parental phenotyping, and fetal platelet counts. We explored the feasibility of allele- specific oligonucleotide probe typing for PIA antigens to determine the risk of second or subsequent fetuses in families where one infant had a diagnosis of anti-PIA1-mediated NATP. A total of eight families at risk for delivering an affected fetus were studied with both serologic and oligonucleotide typing. The correlation between serologic and oligonucleotide PIA types was 100%. Similarly, in an additional eight families not at risk for PIA1-mediated NATP, serologic and oligonucleotide typing maintained a perfect correlation. DNA isolated from fetal leukocytes as well as fetal amniocytes was successfully typed using this technology. Oligonucleotide-based typing of fetuses at risk for NATP whose fathers are heterozygous for the PIA antigens allows early recognition of affected fetuses so that prenatal therapy of mothers can be instituted if necessary. When fetuses are found to be unaffected, invasive, and/or expensive, prenatal interventions can be avoided.