A clinicopathologic study of 11 hemangiopericytomas is reported, and the results of clinical angiography in 4 patients and a correlated microangiographic-histologic study of 1 tumor are presented. Six of the tumors were collected and accepted as hemangiopericytomas in a review of 42 tumors registered as hemangiopericytoma in the Swedish Cancer Registry during the period 1958--1968. The other 5 tumors occurred in patients treated by us. The histologic examination and the follow-up information showed that it can be difficult to predict the prognosis on morphologic grounds, that borderline tumors exist and that a recurrence as well as a metastasis may dedifferentiate. The tumors studied by clinical angiography all proved to be highly vascular. Irregular vessels of varying caliber were filled with contrast medium and, in addition, a prominent diffuse opacification took place. Early filling of veins was not noted despite the high vascularity. The correlated microangiographic-histologic study suggests that the irregular vessels seen on clinical angiograms corresponded to wide, angulated, thin-walled vessels without muscle coat or elastic tissue, while the diffuse opacification was caused by a dense network of delicate, branching, slit-like capillary spaces.