Endothelial cell products, plasminogen activator inhibitor (PAI) and von Willebrand factor (vWF), were assayed in 25 patients with newly-diagnosed and untreated polymyalgia rheumatica (PMR), before and after three and 12 months of corticosteroid treatment. The mean values of PAI and vWF, and also the levels of acute phase reactants (erythrocyte sedimentation rate, fibrinogen, haptoglobin, orosomucoid, C-reactive protein) and platelet counts, were elevated in the active untreated disease. In contrast to the acute phase proteins, both PAI and vWF remained increased after three and 12 months of glucocorticoid treatment. This suggested an active vasculitis, despite a clinically-inactive disease. Particularly high levels of vWF both before and after glucocorticoid therapy were found in two patients who subsequently developed vascular complications during the follow-up period.