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Morphologic features (abnormal mitoses, necrosis, vascular and capsular invasion, broad fibrous bands, cellular pleomorphism, size) previously suggested to be predictors of malignant behavior in adrenal cortical tumors were assessed individually in 23 (17 benign, 6 malignant) pediatric and 42 (29 benign, 13 malignant) adult tumors. Of these features, size was the only predictor of malignancy in pediatric tumors. All pediatric tumors weighing more than 500 g were malignant and all but one weighing less than 500 g were benign. The remaining features were present in both benign and malignant pediatric tumors, and pediatric benign tumors were significantly more likely to have mitoses (P less than 0.01), necrosis (P less than 0.001), broad fibrous bands (P less than 0.005), and moderate to severe pleomorphism (P less than 0.01) than were adult benign tumors. The authors conclude that pediatric tumors are more likely to be benign than previously thought, and that size is the only morphologic predictor of their biologic behavior.