Summary Tumor necrosis factor (TNF) is an important mediator of the host responses to systemic infection. To investigate the effect of TNF on the release of von Willebrand factor (vWF) into the circulation, we performed a controlled study in six healthy men, sequentially measuring the plasma concentrations of vWF antigen after a bolus intravenous injection of recombinant human TNF (50 μg/m2). TNF induced a marked increase in vWF antigen plasma levels, becoming significant after 45 min and peaking after 4 h (percentage increase from base line: 351 ±46; p <0.0001, TNF versus saline). The multimeric organization of circulating vWF was not affected by TNF. We conclude that TNF may be an early mediator of increased vWF secretion in systemic infection.