Background: The incidental discovery of an adrenal mass raises the problem of distinguishing the frequent benign masses from the infrequent malignant ones that require surgery. At present, univocal guidelines to approach this problem are not available. The aim of the present study was to perform a multicentric retrospective analysis of adrenal masses incidentally discovered (adrenal incidentalomas). Patients and Methods: Hospital records of adrenal incidentalomas diagnosed over a 15-year period in 29 surgical and medical centers in Italy were scrutinized. Collected cases were 1,013, and 887 were analyzed. Results: The majority of patients were in the 5th and 6th decade and females were predominantly affected. The frequency of adrenocortical cancer was 12% among operated patients (316 cases). The tumor diameter was highly correlated with the risk of malignancy, as well as the CT characteristics such as density, shape and margins. The frequency of pheochromocytoma was 10% among operated patients. Conclusions: The occurrence of incidentally discovered adrenocortical carcinomas and pheochromocytomas is not rare. Evaluation of the mass size and CT characteristics are simple and effective methods to differentiate malignant lesions. Biochemical screening for pheochromocytoma is mandatory before surgery.