The histopathology of 72 paragangliomas originating in the head and neck region of 71 patients is presented. There were 45 carotid body, 13 vagal body, eight jugulotympanic, and three nasal paragangliomas. In addition, two arose in the larynx and one in the area of the aortic arch. Tumors occurred in four unrelated families. The two most important histologic features leading to a diagnosis of paraganglioma were zellballen and the presence of cytoplasmic argyrophil granules in all cases in which staining with the Grimelius technique was carried out. Ultrastructural study of three carotid body and two vagal body paragangliomas revealed both light and dark chief cells. Tumor cells contained membrane bound, electron dense neurosecretory types of granules, which usually ranged in diameter from 120 to 200 nm. Follow-up information was available for 67 patients (94 per cent). Two of the three nasal paragangliomas, 50 per cent of the jugulotympanic paragangliomas, 17 per cent of the vagal body paragangliomas, and 10 per cent of those of the carotid body recurred locally following attempted surgical resection. All patients treated with radiation had persistent tumor. Four (9 per cent) of the carotid body paragangliomas were malignant, all four patients dying with widespread metastases. One vagal body paragangliomas metastasized to regional lymph nodes (the patient was alive and well at five years), and another caused death by direct intracranial extension. In contrast to the benign tumors, malignant paragangliomas tended to show foci of necrosis and vascular invasion. Mitotic figures, which usually were not identified in the benign cases, were seen in all malignant tumors.