Prediction of biologic behavior in adrenocortical neoplasms is difficult because of the lack of availability of reliable clinical, biochemical, and pathologic prognostic markers. Reliable objective markers predictive of clinical outcome in adrenocortical neoplasms are needed to assign optimal treatment of potentially malignant tumors. In the current article, the authors evaluated a set of molecular markers (topoisomerase II alpha (Topo II alpha), MIB-1, p53, human epithelial cadherin (E-cadherin), retinoblastoma gene protein product, and HER-2/neu) and correlated their expression with histologic diagnosis and clinical outcome. Paraffin-embedded, formalin-fixed tissue blocks from 30 cases of adrenocortical neoplasms (15 benign and 15 malignant) were obtained from the surgical pathology archives at the University of Utah Health Sciences Center (Salt Lake City, UT) and the Medical College of Wisconsin (Milwaukee, WI). Age, gender, recurrence, tumor size and weight, hemorrhage, necrosis, pleomorphism, mitotic count, capsular and lymphovascular invasion, hyaline globules, intranuclear inclusions, and immunohistochemical expression of Topo II alpha, p53, MIB-1, E-cadherin, retinoblastoma gene protein product, and HER-2/neu were studied. Clinical data were obtained from the clinical charts, or communication with the treating physician, or both. Adrenocortical neoplasms with hemorrhage, necrosis, large size (>5 cm), weight more than 100 g, nuclear pleomorphism, lymphovascular invasion, and brisk mitotic rate (more than 5 per 30 high-power fields) were more likely to behave in a malignant fashion (P approximately 0.001-0.009). The difference in proliferation indices in benign and malignant neoplasms was statistically significant (P < 0.001). The difference in p53 staining in benign and malignant neoplasms also was statistically significant (P < 0.001). Higher p53 labeling index (>20%) was present in 73% (11/15) of malignant lesions but was found in only 1 of 15 (6.6%) benign lesions. The difference in retinoblastoma staining between benign and malignant neoplasms was statistically significant (P = 0.004). There was no significant difference in staining pattern of E-cadherin expression between benign and malignant lesions. HER-2/neu overexpression was not observed in any of the benign or malignant adrenocortical neoplasms.