To study the usefulness of calretinin as an immunohistochemistry marker in the diagnosis of cardiac myxoma (CM) and the origin of myxoma cells, we examined 24 CMs and 9 fetal hearts with immunohistochemical methods on formalin-fixed paraffin-embedded tissues. We compared 24 CMs with 10 mural thrombi, 6 jaw myxomas, and 2 papillary fibroelastomas. Calretinin expression was identified in 100% of CMs and was negative in all cases of mural thrombi, jaw myxoma, and papillary fibroelastoma. Calretinin expression by the neoplastic cells in CM was strong and diffuse and had a cytoplasmic and a nuclear pattern. Calretinin expression in fetal hearts was found in autonomic ganglia cells in the subepicardial tissue of the atria and atrial appendages, along the interatrial and atrioventricular sulci, and in the atrial septum. Results clearly indicate that calretinin can be used as a marker for the diagnosis of CM and that it is a powerful tool for the differential diagnosis, most importantly with mural myxoid thrombi. Furthermore, the positive expression of calretinin by the autonomic neurons in the fetal heart and CM supports the concept that myxoma cells may originate from endocardial sensory nerve tissue.