—Osteopontin (OPN), an extracellular matrix protein, is expressed in the myocardium with hypertrophy and failure. We tested the hypothesis that OPN plays a role in left ventricular (LV) remodeling after myocardial infarction (MI). Accordingly, OPN expression and LV structural and functional remodeling were determined in wild-type (WT) and OPN knockout (KO) mice 4 weeks after MI. Northern analysis showed increased OPN expression in the infarcted region, peaking 3 days after MI and gradually decreasing over the next 28 days. In the remote LV, OPN expression was biphasic, with peaks at 3 and 28 days. In situ hybridization and immunohistochemical analyses showed increased OPN mRNA and protein primarily in the interstitium. Infarct size, heart weight, and survival were similar in KO and WT mice after MI (P=NS), whereas the lung wet weight/dry weight ratio was increased in the KO mice (P<0.005 versus sham-operated mice). Peak LV developed pressure was reduced to a similar degree after MI in the KO and WT mice. The number of terminal deoxynucleotidyl transferase–mediated dUTP nick end-labeling (TUNEL)-positive myocytes was similar in KO and WT mice after MI. In contrast, post-MI LV chamber dilation was approximately twice as great in KO versus WT mice (P<0.001). Myocyte length increased after MI in WT mice (P<0.001) but not in KO mice. Electron microscopy showed increased collagen content in WT mice after MI but not in KO mice after MI. Type I collagen content was increased ≈3-fold and ≈7-fold in remote and infarcted regions, respectively, of WT hearts after MI but not in KO hearts (P<0.01 versus WT hearts). Likewise, Northern analyses showed increased collagen I(α₁) mRNA after MI in remote regions of WT hearts but not in KO hearts. Thus, increased OPN expression plays an important role in regulating post-MI LV remodeling, at least in part, by promoting collagen synthesis and accumulation.