The information concerning aneurysmal progress in Behcet's disease is still insufficient, while researches in the role of matrix metalloproteinases (MMPs) in aneurysmal formation are rapidly expanding. The goal of the present study is to investigate the role of metalloproteinase 9 (MMP-9) in vascular complications which is observed in 10% of Behcet's disease patients. Three groups have been studied; patients with Behcet's disease, patients with Behcet's disease who have vascular problems (vasculo-Behcet's), and patients with abdominal aortic aneurysm (AAA). The third group was used as a control. The activity and gene expression levels of MMP-9 in plasma have been determined. We showed that compared to AAA patients there was no difference in the MMP-9 activity in Behcet's disease patients (vascular and non-vascular). We also evaluated the gene expression level and activity of MMP-9 for every patient. The increase in the gene expression level for MMP-9 could only be detected at two patients. One of them was Behcet's, the other was AAA patient. It is surprising that MMP levels of these patients were different. While the patient with Behcet's had low protein level, another patient with AAA had high of MMP-9 level. This result suggested to us that the relationship between gene expression and active protein level is not correlated. It is not sufficient alone to determine MMPs levels for evaluating the pathogenesis. At the same time gene expression and the level of active protein should be assessed together.