An indefinitely formed heart is a compound lesion of the cardiovascular system in which an anatomical examination of the atria does not permit any decisive interpretation of the formation of the heart. The disease can be subdivided into two pathological syndromes. The first syndrome includes a partially patent atrioventricular canal with a joint atrium, an absence of the hepatic segment of the inferior vena cava, a partally anomalous drainage of the pulmonary veins, bilobular lungs, abdominal heterotaxy and polysplenism. The second syndrome is characterized by an open atrioventricular canal with a joint atrium or a cor biloculare, an abberant superior vena cava, or varilateral position of the superior and inferior venae cavae, a completely anomalous drainage of the pulmonary veins, transposition of the major vessels with a stenosis or atresia of the pulmonary artery, trilobular lungs, abdominal heterotaxy and asplenism. The leading role in the diagnosis of the pathology belongs to heart catheterization and angiocardiography.