Platelets from a patient with a suspected case of posttransfusion purpura were subjected to alloantigen phenotyping and found to express the PlA1, but not the PlA2, allelic form of human platelet membrane glycoprotein (GP) IIIa on the platelet surface. However, genotyping showed unambiguously that the patient carried the genes for both of these GPIIa alleles. Based on these results, we postulated that the PlA2 allele was silent, ie, that this patient was a carrier for Glanzmann thrombasthenia (GT). Quantitative analysis of GPIIb-IIIa surface expression showed only 20,000 GPIIb-IIIa receptors/platelet, approximately half of the value obtained with control platelets. Southern blot analysis showed no large deletions or insertions within the GPIIIa gene, and amplification of all 14 exons encoding GPIIIa resulted in the production of normal sized polymerase chain reaction (PCR) products in all cases. DNA-sequence analysis showed an AG dinucleotide deletion affecting codons 210 and 211 within exon 4 of the GPIIIa gene, leading to a change in reading frame and the creation of a stop codon 38 nucleotides down-stream. The predicted truncated protein consists of only the first 223 of the normal 762 amino acids, thus accounting for the failure to express the PlA2 allele on the platelet surface. While encountered only rarely, carriers of either GT or Bernard Soulier syndrome that are at the same time heterozygous for human platelet alloantigenic epitopes found on GPIb, GPIIb, or GPIIIa have the possibility to give discrepant results when comparing genotypic versus phenotypic analysis. In such situations, the combination of serologic and DNA-based evaluation contributes complementary and beneficial diagnostic information than either one alone are able to provide.