AbstractThis study examined the effects of platelet‐derived serum factors from patients with idiopathic thrombocytopenic purpura (ITP) on human umbilical cord vascular endothelial cell growth and DNA synthesis. Vascular endothelial cell growth factors were identified in the platelet extracts, and platelet‐derived vascular endothelial cell growth factors with molecular weights of less than 10,000 were also identified in the sera. These growth factors were stable even after 30 minutes of heat treatment at 56°C and acid treatment.When ITP patient serum was added to vascular endothelial cells, their growth capacity and DNA synthesis capacity were lower than when serum from normal subjects or patients with ITP in remission was added. This indicated that in ITP, impairment of vascular integrity and abnormal repair of the vascular walls that accompany thrombocytopenia may be causes of the predisposition to hemorrhage. The decrease in ITP of platelet‐derived vascular endothelial cell growth factors with molecular weights of less than 10,000 was hypothesized to be a contributory