Summary This report concerns an incidental finding during slaughter inspection of the heart of a heifer from a breeding herd. The lesion, studied by histopathology and immunohistochemistry, was found to be composed of different types of cardiac tissues, all immature, disordered and disproportionate. Most conspicuous was a dispersed population of large, vacuolated, PAS-positive cells, forming islands, enveloped in excessive fibrous connective tissue. These cells were identified as abnormal Purkinje fiber-like cells, known as spider cells, which were markedly positive for desmin and negative for vimentin, α-smooth muscle actin, and myogenin factor 4. Based on the characteristic changes and immunoreactivity, the lesion was originally classified as a cardiac rhabdomyoma. The equivalence of this lesion with cardiac hamartoma is intrinsic, since cardiac rhabdomyomas are considered not a neoplasm but congenital hamartomas. However, in contrast to cardiac hamartomas in animals, the main component of which is an abnormal vascular pattern, which was not a main feature in this lesion, hamartoma did not fit the description. The neovascularization and hypertrophy of the tunica media in the arteries, recognized in this case, were probably a compensatory adaptation of the myocardium, but appear largely different from reported cases of hamartomas, both in animals and humans. Another abnormal tissue component was extensive replacement of the myocardium with mature fibrous and fatty tissues. This was interpreted as clear evidence of dysplasia. Furthermore, abnormal cardiomyoblasts organized into tortuous bundles were the last tissue component. However, these cells displayed distinctive striations and even intercalated discs. Some of these abnormal myoblasts...... middle of paper ......n BJE (1994) Congenital cardiac rhabdomyomas in Red Wattle pigs. Can Vet J, 35, 48-49Radi ZA, Metz A (2009) Canine cardiac rhabdomyoma. Toxicological Pathology, 37, 348-350. Sugiyama A, Ozaki K, Takeuchi T, Narama I (2007) Cardiac vascular hamartoma in wo slaughtered cattle. Journal of Comparative Pathology, 136, 202-205. Tanimoto T, Ohtsuki Y (1995) The pathogenesis of so-called cardiac rhabdomyoma in pigs: a histological, immunohistochemical and ultrastructural study. Virchows Arch, 427, 213-221.Travis WD, Brambilla E, Muller-Hermelink HK, Harris CC (eds) World Health Organization Classification of Tumors. Pathology and genetics of tumors of the lung, pleura, thymus and heart. IARC Press: Lyon 2004.Uzun O, Wilson DG, Vujanic GM, Parsons JM, De Giovanni JV (2007) Cardiac tumors in children. Orphanet Journal of Rare Diseases, 2, 11 doi:10.1186/1750-1172-2-11.