Forty-eight surgical specimens were examined histologically and the case histories reviewed to determine the histopathologic features of embolized paragangliomas (glomus tumors) in relation to the time interval between embolization and surgery. Different degrees of thrombus formation and of multinucleated foreign body giant cells occurred during the first 7 days after embolization; thereafter, glant cells with active phagocytosis, fragmentation of embolic material, and partial revascularization were observed. Only one third of the tumor vessels were embolized. Complete obliteration of 40% of embolized vessels occurred more than 2 months postembolization. The histologic changes induced by embolization in paragangliomas of the head and neck may be classified in four stages. Histologically, the best time for surgery is within 8 days from embolization; surgery performed more than 8 days following embolization, however, is not compromised by revascularization.