Previous investigations have demonstrated an increased release of von Willebrand factor (VWF; also known as vWF) in endothelial cells after high single-dose irradiation in vitro. We have also found increased levels of Vwf protein in mouse glomeruli after a high single dose of renal irradiation in vivo. In addition, increased numbers of leukocytes were observed in the renal cortex after irradiation in vivo. The aim of the present study was to investigate and quantify these biological processes after clinically relevant fractionated irradiation and to relate them to changes in renal function. A significantly greater increase in release of VWF was observed in cultured human umbilical vein endothelial cells (HUVECs) after fractionated irradiation (20 x 1.0 Gy) than after a single dose of 20 Gy (147% compared to 115% of control, respectively, P < 0.0005). In contrast with the in vitro observations, glomerular Vwf staining was lower after fractionated irradiation in vivo (20 x 2.0 Gy or 10 x 1.6 Gy +/- re-irradiation) than after a single dose of 16 Gy. The number of leukocytes accumulating in the renal cortex was also lower after fractionated in vivo irradiation than after a single radiation dose. The onset of these events preceded renal functional and histopathological changes by approximately 10 weeks. These data indicate that radiation-induced changes in endothelial VWF expression after in vivo irradiation may be distinct from the in vitro observations. Increased VWF expression may reflect pivotal processes in the pathogenesis of late radiation nephropathy and provide a clue to appropriate timing of pharmacological intervention.