BACKGROUND: In approach to an adrenal incidentaloma, early exclusion of pheochromocytoma is clinically important, due to the risk of catecholamine crisis. The aims of this study are to investigate the characteristics of incidentally detected pheochromocytomas, compared with that of the other adrenal incidentalomas, and to compare these characteristics with those of symptomatic pheochromocytomas. METHODS: In this retrospective study, we reviewed the medical records of 198 patients with adrenal incidentaloma from 2001 to 2010. We analyzed the clinical, laboratory and radiological data of pheochromocytomas, in comparison with those of the other adrenal incidentalomas. We also compared the characteristics of these incidentally detected pheochromocytomas with the medical records of 28 pathologically proven pheochromocytomas, diagnosed based on typical symptoms. RESULTS: Among the 198 patients with adrenal incidentaloma, nineteen patients were diagnosed with pheochromocytoma. Pheochromocytomas showed larger size and higher Hounsfield unit at precontrast computed tomography (CT) than did non-pheochromocytomas. All pheochromocytomas were larger than 2.0 cm, and the Hounsfield units were 19 or higher in precontrast CT. When both criteria of size > 2.0 cm and Hounsfield unit > 19 were met, the sensitivity and specificity for the diagnosis of pheochromocytoma were 100% and 79.3%, respectively. Compared with patients with pheochromocytoma, diagnosed based on typical symptoms, patients with incidentally detected pheochromocytoma were older, presented less often with hypertension, and showed lower levels of 24-hour urine metanephrine. CONCLUSION: Adrenal incidentaloma with < 2.0 cm in size or < or = 19 Hounsfield units in precontrast CT imaging was less likely to be a pheochromocytoma. Patients with incidentally discovered pheochromocytoma showed lower catecholamine metabolites, compared with those patients with symptomatic pheochromocytoma.