Matrix metalloproteinase-9 (MMP-9) and tissue inhibitor of metalloproteinase-1 (TIMP-1) play important roles in the function of the blood-brain barrier (BBB). To investigate the function of the BBB during tick-borne encephalitis (TBE), the levels of MMP-9 and its common tissue inhibitor, TIMP-1, were measured in serum from patients with acute phase of TBE.