The purpose of this study was to investigate the role of matrix metalloproteinase-9 (MMP-9) in the pathogenesis of vascular leakage in patients with dengue virus infection. Serum samples from 24 children with serologically confirmed dengue virus infection (dengue fever [DF], 16; dengue hemorrhagic fever [DHF], 8; age, 9.5+/-2.4 years; 67% male] were analyzed for MMP-9 during the febrile and toxic stages and at follow-up. Serum samples obtained from 7 healthy children were used as controls. Serum MMP-9 levels in patients with dengue virus infection were found to be lower at the febrile (227.0+/-186.9 ng/ml) and toxic stages (150.9+/-151.7 ng/ml) than at follow-up (424.5+/-227.8 ng/ml) or in the control group (393.3+/-125.9 ng/ml, P<0.001 by one-way ANOVA). There was no significant difference between MMP-9 levels in patients with DHF and those with DF at any stage of the disease. In conclusion, MMP-9 levels are reduced during the febrile and toxic stages of dengue virus infection.