Intraluminal formations of the heart are a heterogeneous group: from blood clots to malignant neoplasms. The nosology of these formations can only be determined by morphological research. Myxofibrosarcoma of the heart is a rare malignant tumor. According to modern concepts, myxofibrosarcoma belongs to the group of intimal sarcomas, subgroup of undifferentiated pleomorphic sarcomas. We describe two cases of myxofibrosarcoma of the heart: in one subject against the background of inflammatory myofibroblastic heart tumor; in another subject - with a history of breast cancer and diffuse large B-cell lymphoma. An attempt was made to identify similar mutations in patients with these tumors according to the literature.