Background-One of the hallmarks of dilated cardiomyopathy (DCM) is left ventricular (LV) remodeling. The matrix metalloproteinases (MMPs) are a family of enzymes that contribute to extracellular remodeling in several disease states. Additionally, a family of inhibitors called tissue inhibitors of MMPs (TIMPs) has been shown to exist and to tightly regulate MMP activity. However, the types of MMPs and TIMPs expressed within the normal and DCM LV myocardium and the relation to MMP activity remain unexplored. Methods and Results-Relative LV myocardial MMP activity was determined in the normal (nϭ8) and idiopathic DCM (nϭ7) human LV myocardium by substrate zymography. Relative LV myocardial abundance of interstitial collagenase (MMP-1), stromelysin (MMP-3), 72 kD gelatinase (MMP-2), 92 kD gelatinase (MMP-9), TIMP-1, and TIMP-2 were measured with quantitative immunoblotting. LV myocardial MMP zymographic activity increased with DCM compared with normal (984Ϯ149 versus 413Ϯ64 pixels, PϽ.05). With DCM, LV myocardial abundance of MMP-1 decreased to 16Ϯ6% (PϽ.05), MMP-3 increased to 563Ϯ212% (PϽ.05), MMP-9 increased to 422Ϯ64% (PϽ.05), and MMP-2 was unchanged when compared with normal. LV myocardial abundance of TIMP-1 and TIMP-2 increased by Ͼ500% with DCM. A high-molecular-weight immunoreactive band for both TIMP-1 and TIMP-2, suggesting a TIMP/MMP complex, was increased Ͼ600% with DCM. Conclusions-This study demonstrated increased LV myocardial MMP activity and evidence for independent regulatory mechanisms of MMP and TIMP expression with DCM. These findings suggest that selective inhibition of MMP species within the LV myocardium may provide a novel therapeutic target in patients with DCM.