No comprehensive series has evaluated the histologic features of pheochromocytoma to separate benign from malignant pheochromocytoma by histomorphologic parameters only. Fifty histologically malignant and 50 histologically benign pheochromocytomas of the adrenal gland were retrieved from the files of the Armed Forces Institute of Pathology. The patients included 43 females and 57 males, with an age range of 3-81 years (mean 46.7 years). Patients usually experienced hypertension (n = 79 patients). The mean tumor size was 7.2 cm (weight was 222 g). Histologically, the cases of malignant pheochromocytomas of the adrenal gland more frequently demonstrated invasion (vascular [score = 1], capsular [score = 1], periadrenal adipose tissue [score = 2]), large nests or diffuse growth (score = 2), focal or confluent necrosis (score = 2), high cellularity (score = 2), tumor cell spindling (score = 2), cellular monotony (score = 2), increased mitotic figures (>3/10 high power fields; score = 2), atypical mitotic figures (score = 2), profound nuclear pleomorphism (score = 1), and hyperchromasia (score = 1) than the benign tumors. A Pheochromocytoma of the Adrenal gland Scaled Score (PASS) weighted for these specific histologic features can be used to separate tumors with a potential for a biologically aggressive behavior (PASS > or =4) from tumors that behave in a benign fashion (PASS <4). The pathologic features that are incorporated into the PASS correctly identified tumors with a more aggressive biologic behavior. Application of these criteria to a large cohort of cases will help to elucidate the accuracy of this grading system in clinical practice.