There was a markedly significant increase of plasma levels of tissue-type plasminogen activator, plasminogen activator inhibitor 1 (PAI-1) and PAI activity in 3 patients with hemophagocytic histiocytosis (HPH) as compared with DIC patients with multi-organ failure (MOF). Of particular interest was the peculiar increase of PAI-1 levels, that is, the average PAI-1 level was 2,673 ng/ml, while that in DIC patients with MOF was 66 ng/ml. We conclude that the striking increase of plasma PAI-1 levels may be a pathognomonic feature of HPH and may contribute to the pathogenesis of DIC and/or MOF associated with HPH.