BACKGROUND: Nonfunctioning adrenal incidentalomas are common and many patients undergo adrenalectomy to exclude adrenocortical carcinoma (ACC). Recent studies have shown dysregulated microRNA (miRNA) expression in ACC. The objective of this study was to determine the feasibility and diagnostic accuracy of measuring serum miRNAs in patients with benign and malignant adrenocortical neoplasms. METHOD: Five miRNAs were selected from miRNA profiling studies in ACC (miR-let-7d, -34a, -195, -214, and 483-5p). Total miRNA was extracted from serum samples in patients with malignant and benign adrenal neoplasms. miRNAs levels were measured by quantitative reverse transcript polymerase chain reaction and normalized to miR-16. To determine if miRNAs were secreted from ACC cells, we measured miRNA levels in culture. RESULTS: Serum samples from 22 patients with cortical adenomas and 17 patients with ACC were analyzed, and all 5 miRNAs were detected. We found greater levels of miR-34a (P = .001) and miR-483-5p (P = .011) in patients with ACC. The area under the receiver operating characteristic curve was 0.81 for miR-34a and 0.74 for miR-438-5p. MiR-34a and miR-483-5p levels in ACC cells were greater in the supernatant at 48 hours compared with intracellular levels. CONCLUSION: We show that dysregulated miRNAs in ACC are detectable in human serum samples. MiR-34a and miR-483-5p are candidate serum biomarkers for distinguishing between benign and malignant adrenocortical tumors.