SINCE THE introduction of the term "hemangiopericytoma" by Stout and Murray1 in 1942, the ubiquitous nature of this tumor has been documented by numerous reports.2-7 However, the arteriographic features of this lesion have been described only rerely8-11 and its physiologic behavior has not been investigated. The purpose of this report is to emphasize these aspects of this highly vascular tumor which in one case functioned as a large arteriovenous shunt. Report of a Case A 78-year-old woman was admitted to Montefiore Hospital in December 1965 with a large soft tissue mass on the medial aspect of the left thigh. This mass, present for more than ten years, had begun to grow markedly during the last year. The patient was totally asymptomatic except for a recent 4.5 kg (10 lb) weight loss, and she had never had any previous hospitalization or major illness. Physical examination demonstrated a thin,