Endoglin is an 180 KDa protein which plays an important role in the vascular system and cardiac embryogenesis. Indeed, monoallelic mutations in the endoglin gene are associated with the development of hereditary hemorrhagic telangiectasia type 1; moreover endoglin knockout mice die precociously because of severe arteriovenous and cardiac malformations. In this study, endoglin immunohistochemical expression was analyzed in the heart of 23 fetuses (9-38 weeks), 5 of which displayed cardiac malformations, as well as in cardiac samples from 4 preterm and 1 term infants. At the ninth week, endoglin expression was recorded in the endocardium; it extended into the epicardial and myocardial vessels by the 10th week. This pattern was maintained throughout gestation in most fetuses, but not in those with cardiac malformations. Endoglin expression up to term gestation indicates that its role in human heart development is not limited to the early gestation. Endoglin altered expression in association with cardiac defects further highlights its importance in normal cardiac embryogenesis and morphogenesis.