Despite the absence of enzymes that digest extracellular matrix, Borrelia burgdorferi spreads in the skin to form erythema migrans (EM) lesions and then disseminates to other organs. We studied the induction by bacteria of host matrix metalloproteinases (MMPs) in EM skin lesions of patients with acute Lyme disease. In blister fluid from the EM lesions, the expression of MMP-9 was selectively increased by 1900%+/-1037%, compared with blister fluid from the surrounding normal-appearing skin. The expression of all other MMP messenger RNAs was similar in the EM lesions and normal-appearing skin. Selective up-regulation of MMP-9 in the EM lesions was found. Fibroblasts and, to a lesser degree, mononuclear cells were the sources of local MMP-9 production. These results demonstrate specific up-regulation of MMP-9 in the EM skin lesions of patients with acute Lyme disease. Bacterial induction of host proteases may play a role in the dissemination of B. burgdorferi.