The clinical and pathologic features of 60 adrenal pheochromocytomas were reviewed in an attempt to evaluate the utility of histopathologic evaluation in predicting the prognosis for these tumors. Fifty-five tumors were benign, and five were malignant, characterized either by histologically proved metastases or by extensive local invasion. The prevalence of all parameters studied was compared between the benign and malignant groups. Three differences were observed between the benign and malignant tumors. The malignant tumors were usually larger, had extensive areas of necrosis, and were composed of small cells. In agreement with the results of previous investigations, the morphologic criteria generally used to predict the behavior of tumors, i.e., nuclear atypia, capsular and vascular invasion, and mitotic activity, were of little value in predicting the behavior of adrenal pheochromocytomas. Fifteen tumors, all of which had membrane-bound, cytoplasmic granules, were examined electron microscopically. A minority of these tumors fit the classic descriptions of "norepinephrine" and "epinephrine" granules reported in the literature, while the majority of the granules had features of both types, precluding definitive classification. As a result of these observations, the currently accepted criteria of norepinephrine and epinephrine granules were questioned. A review of the literature cast further doubt on the existence of a correlation between granule content and morphology. Both of the patients with locally invasive malignant tumors were alive and well eight and 28 years following diagnosis and radical surgery. In contrast, the patients with histologically proved distant metastases died within one year of diagnosis. These observations suggest that locally invasive tumors may not have the same dismal prognosis as adrenal pheochromocytomas that have metastasized.