Sinonasal hemangiopericytomas are rare, occasionally misdiagnosed neoplasms that have often been considered distinct from hemangiopericytomas of other sites. Eleven cases were studied. Nine arose from the nasal cavity and two from the paranasal sinuses. The patients' mean age was 58 years. In nine cases there were no appreciable mitoses; in two, mitoses were frequent. Of 10 cases studied by immunostaining, all were positive for vimentin; two had faint focal staining for actin; one focally expressed S-100 protein; all were negative for cytokeratins, desmin, and Factor VIII-related antigen; and none bound Ulex europaeus agglutinin 1. Five cases were studied by electron microscopy. The most consistent features were basal lamina-like material partly surrounding tumor cells and completely separating them from endothelium, tapered cytoplasmic extensions, and orderly bundles of filaments. Intercellular junctions and pinocytotic vesicles were present in some tumors. Of nine cases with adequate follow-up, tumors recurred in four cases (44%) after a mean of 6.5 years, and none metastasized. A review of the literature showed that high local recurrence rates, late recurrences, and low rates of metastasis were features of tumors in this location. This might be a reflection of early presentation, small tumor bulk, and difficulty of complete resection, rather than evidence for a biologically distinct neoplasm.