Lumican belongs to the small leucine-rich proteoglycan family and has an important role in the regulation of corneal and dermal collagen fiber assembly. Recently, lumican mRNA was found in the heart and its high expression was reported during wound healing of the cornea. In the present study, the expression and role of lumican in fibrosis of an ischemic and reperfused rat heart were examined. The expression level of lumican mRNA increased in the ischemic and reperfused rat heart and was highest on the fourth week. Lumican protein existed in the forms of core protein and proteoglycan in the control rat heart. The amount of lumican in the form of proteoglycan increased, and that as the form glycoprotein was newly detected in the ischemic and reperfused rat heart on the fourth week. In the control heart, lumican was weakly expressed in the collagen fibers of the perivascular area, but it was expressed strongly in many capillary endothelial cells in the ischemic lesion on day 1. After 3 days, lumican was localized in collagen fibers and in the fibroblasts of fibrotic lesions. A few myocardial cells close to the ischemic lesion expressed lumican mRNA. Lumican is considered to play an important role in the fibrillogenesis of the ischemic and reperfused rat heart. (Jpn Circ J 2001; 65: 445 - 450)