We investigated plasma levels of plasminogen activator inhibitor 2 (PAI-2) in 40 patients with hematological malignancies including acute leukemia, chronic leukemia, hemophagocytic histiocytosis (HH) and histiocytic sarcoma (HS). The plasma PAI-2 levels in the patients with mononuclear phagocyte system (MPS) proliferative disorders (M4, M5, and CMMoL on FAB classification, HH and HS) were all above the cut-off value in healthy subjects. In a patient of reactive HH, there was a close correlation between Plasma PAI-2 and serum ferritin levels during the clinical course. These results indicate that the increase of plasma PAI-2 would be a diagnostic indicator of MPS proliferative disorders.