We studied 20 families with Glanzmann thrombasthenia, including 20 parents and 22 siblings, for carrier detection. Carrier detection was done by phenotypic analysis (flow cytometry and Western blot) and direct gene analysis (sequencing or restriction fragment length polymorphism). Reduced expression of the glycoprotein IIb/IIIa complex was found by flow cytometry in 17 (85%) of the parents and 12 (55%) of the siblings. Western blot showed a reduced or an abnormal band in 6 (30%) of the parents and 8 (36%) of the siblings. DNA analysis in family members showed all parents and 16 (73%) of siblings to be carriers. Both techniques, flow cytometry and Western blot, were evaluated with respect to DNA analysis as a "gold standard" method. The sensitivity of flow cytometry was higher (75%) than that of Western blot (39%). We concluded that flow cytometry can effectively be used for carrier detection in Glanzmann thrombasthenia.