Several reports have described the presence of giant platelets in patients with type 2B von Willebrand disease (VWD). We have now characterized the ultrastructural changes in platelets from three unrelated patients with type 2B VWD and different mutations within exon 28 of the von Willebrand factor (VWF) gene. Electron microscopy showed that each of these subjects had an increased proportion of large platelets when compared with those of a patient with type 2A VWD or control subjects. Immunogold labelling for VWF was performed. Large masses detected by anti-VWF antibody were seen not only on the platelet surface, but also inside the platelet surface-connected canalicular system (SCCS) when ultrathin sections were labelled. This suggested translocation of the abnormally bound VWF from the platelet surface. Labelling of the alpha-granules was eccentric as for normal platelets. Labelling for glycoprotein (GP) Ib was seen on the surface and within the SCCS, suggesting co-localization with the bound VWF. However, there was no evidence for VWF in endosomes or other endocytic vesicles. The presence of platelet-bound VWF was not accompanied by high levels of platelet activation, as detected by electron microscopy, or by using monoclonal antibodies against P-selectin or activation-dependent determinants on GP IIb-IIIa in flow cytometry. Intriguingly, platelet ultrastructure often resembled that seen in patients with congenital thrombocytopathies characteristic of giant platelet syndromes.