Twelve cases of apparently primary thoracic hemangiopericytoma are reported: 7 intrapulmonary and 5 extrapulmonary. These cases, taken together with 75 others already published, serve as a basis to describe the characteristics of thoracic hemangiopericytomas. Intrapulmonary forms raise the problem of a single peripheral X-ray lesion, often detected in a routine film, with no accompanying endoscopic abnormality. Extrapulmonary forms also take the form of a well defined tumor with diaphragmatic or mediastinal parietal connections. Up to the present, the diagnosis has always been made at thoracotomy. It is probable that a preoperative diagnosis will be possible in the future by transbronchial biopsy. Silver stains and the use of the electron microscope form the basis of histological diagnosis. However it is not possible to distinguish by histology between benign and malignant forms, nor between primary or metastatic hemangiopericytomas. It is for this reason that doubt will long persist as to the primary nature of the tumor and only prolonged survival of patients after excision confirms that the lesion was indeed primary. Treatment is essentially surgical. New high energy radiotherapy techniques and new possibilities in chemotherapy (using adriamycin) should improve the prognosis in those forms which run a malignant course. However the standard therapeutic strategy for these rare tumors has yet to be defined.