Background: The common histopathology of Behcet's disease is vasculitis associated with activation of neutrophils. The level of plasma elastase - α1 - proteinase inhibitor (E - α1 - PI), which represents the activation of neutrophils, may be a marker of Behcet's disease. Objective: We examined the level of plasma elastase - α1 - proteinase inhibitor to evaluate the degree of neutrophil activation in Behcet's disease. Method: We measured plasma elastase - α1 - proteinase inhibitor in 34 eases of untreated Behcet's disease patients and 30 cases of normal individuals by an enzyme immunoassay. We also studied the differences between the levels in two clinical types of Behcet's disease, the complete and incomplete type. Results: The plasma leve1 of elastase - α1 - proteinase inhibitor was significantly higher in untreated Behcet's disease patients than in healthy controls. However, there was no significant difference between the levels in the two clinical types. Conclusion:These data suggest that the elevated level of plasma elastase - α1 - proteinase inhibitor may reflect a state of chronic activation of neutrophils in Behcet's disease immunologically and further studies will be needed to evaluate the clinical status of Behcet's disease patients by measuring levels of plasma elastase - α1 - proteinase inhibitor. (Ann Dermatol 11(1) 9~12,1999).