In the haemolytic uraemic syndrome (HUS), platelet microthrombi and deposition of fibrin in glomeruli may contribute to the development of renal failure. The balance of procoagulant and fibrinolytic activities in the renal vasculature may therefore have an important role. We measured plasminogen-activator inhibitor (PAI) activity in the plasma of 81 children with diarrhoea-associated (D+) HUS, and found elevated PAI activity in many patients. When we categorized patients by need for dialysis, only the dialysed group had significantly higher levels of activity, compared with a group of normal controls. We also compared PAI activity with patient outcome after one year, and found that those with a poor outcome had significantly higher PAI activity than those with a good outcome. We suggest that plasma PAI activity may be an acute marker for dialysis requirement in HUS, and may have prognostic value for long-term outcome. The possible role of PAI in the pathogenesis of HUS requires further investigation.