Bernard-Soulier syndrome (BSS) is a rare inherited disorder with giant platelets, thrombocytopenia and a prolonged bleeding time. These abnormalities are caused by genetic defects of the glycoprotein (GP) Ib-IX complex that constitutes the von Willebrand factor receptor on the platelet surface. Here, we describe four unrelated German patients with low platelet counts and mild bleeding tendency. Three patients had been diagnosed with immune thrombocytopenia (ITP) and were treated with steroids without response. Another patient presented with easy bruising. Peripheral blood smears showed giant platelets. Ristocetin-induced platelet aggregation was almost absent, and quantitative flow cytometry and Western blotting disclosed a greatly reduced surface expression of GPIb-IX. Unexpectedly, sequencing the entire coding regions of GPIbalpha, GPIbbeta and GPIX revealed that all four unrelated patients were homozygous for an A to G mutation in position 1826 of the GPIX gene, constituting a Asn-45Ser change. This mutation has been described before and now represents by far the most often identified molecular defect causing BSS in Caucasians. Because BSS patients are likely to be misdiagnosed with ITP, treatment-resistant ITP patients should be re-evaluated thoroughly. Asn-45Ser genotyping may be a helpful tool for differential diagnosis.