The clinical and pathologic features of 120 adrenal and extraadrenal paragangliomas were studied in an attempt to identify features which might predict malignant behavior. Clinical follow-up was obtained in 98 cases (82%); 64 tumors were clinically benign, and 34 were malignant as evidenced by regional or distant metastases and/or extensive local invasion. Thirty-two of the 34 malignant tumors (94%) were functionally active. Features noted more frequently in malignant tumors included male predominance (74%; P2 [two-sided P value] = .002), extraadrenal location (52%; P2 less than .0001), greater tumor weight (mean 383 g versus 73 g for nonmalignant tumors), confluent tumor necrosis, and the presence of vascular invasion and/or extensive local invasion. Intracytoplasmic hyaline globules were seen in 59% and 32% of benign and malignant tumors, respectively (P2 = .001). Logistic regression analysis of 16 nonhistologic and histologic parameters showed four of them to be most predictive of malignancy--extraadrenal location, coarse nodularity of the primary tumor, confluent tumor necrosis, and absence of hyaline globules. Most malignant paragangliomas had two or three of these features (71%), while 89% of benign tumors had only one (or none; P less than .0001). According to the statistical model developed, there was better than a 95% probability that more than 70% of tumors could be classified correctly on the basis of the four factors indicated. Although limitations still exist, results of this study provide some basis for evaluating malignant potential of these tumors.