Recent evidence suggests that a key feature in the pathogenesis of hypertrophic osteoarthropathy (HOA) is an enhanced local platelet vessel wall interaction in the affected extremities. In our investigation we measured plasma levels of von Willebrand Factor Antigen (vWF:Ag; ELISA assay), a marker of platelet and/or endothelial activation, in 5 patients with the primary form of HOA and in 6 patients with the secondary form (due to cyanotic heart disease). Seven subjects matched for sex and age were used as controls. Patients showed statistically significantly higher levels of vWF:Ag in the primary (163.8 +/- 13.9%) and secondary form (152.6 +/- 9.6%, p less than 0.001 for both) when compared to controls (100.4 +/- 6.3%). Our results support the notion that platelet endothelial cell interaction may play a key role in the development of HOA.