Extramedullary hematopoiesis is a rare finding. It represents a compensatory mechanism capable of assuring and keeping sufficient erythrogenesis. The distribution of the main locations of extramedullary hematopoiesis was observed in different organs and apparatuses. According to their experience, the authors investigated the potentials and limitations of conventional radiology, of CT and MR Imaging, to diagnose intrathoracic extramedullary hematopoiesis in the posterior mediastinum. While stressing the value of clinical and anamnestic data and of conventional radiology, the authors emphasize the leading role of CT and MR imaging. As a matter of fact, the latter is the method of choice, thanks to higher contrast resolution and to its multiplanar and multiparameter capabilities, in the study of space-occupying lesions in the costo-vertebral region. On the other hand, MR Imaging fails to detect calcic areas in paravertebral masses; moreover, the exhaustive and morphostructural analysis of bone segments, ribs and vertebrae, is allowed only by CT.