The detection of an adrenal mass as an incidental finding has become so frequent that this trend is considered to be an endocrine epidemic. In these adrenal incidentalomas, a difficult problem is to distinguish between benign and malignant primary or secondary tumors. Another major problem in the management of patients with incidentally discovered adrenal mass is whether and when the mass puts the patient at increased risk of an adverse outcome. A Japanese government-supported study conducted by Professor Miyachi's research committee reviewed the records of all university hospitals and hospitals possessing more than 200 beds in Japan and found 2106 Japanese patients with adrenal incidentaloma during the year 1999. They comprised 1095 males (52%) and 1011 females (48%), mostly distributed between 50 and 59 years of age. The tumor size ranged mostly from 1.1 to 2.0 cm. Almost half (50.3%) of all the incidentalomas were diagnosed as non-functional adenomas, and 5.6% were primary or secondary malignant tumors. In conclusion, accurate assessment of tumor size is an important consideration during preoperative evaluation of adrenal tumors in order to rule out malignancy. Careful evaluation of endocrine functions of the hypothalamic-pituitary-adrenal axis is necessary before surgical treatment in patients with adrenal incidentaloma. Patients with adrenal incidentaloma should be carefully followed, especially regarding size and hormonal conditions.