Incidental adrenal masses are typically found during radiologic imaging of kidneys and adrenals. They are classified as hormonally active or nonfunctioning, and as malignant or benign. Biochemical and imaging evaluations are required to determine this taxonomy.  Adrenal masses are among the most common tumors in humans. The prevalence of clinically inapparent adrenal masses increases with age, being <1% for patients younger than 30 years of age and increasing to 7% in patients 70 years of age and older. 
Prevalence data on the etiology has found that 41% are adenomas, 19% metastases, 10% adrenocortical carcinomas, 9% myelolipomas, up to 8% pheochromocytomas, and the remainder other mostly benign tumors (cysts, adrenolipomas, hematomas, leiomyomas, lymphangiomas).  
The prevalence of adrenal adenomas is 6% based on autopsy studies, and 4% based on abdominal CT.  Most hormonally active adrenal tumors causing Cushing syndrome, pheochromocytoma, or primary hyperaldosteronism are benign, but uncommonly they may be malignant. However, adrenocortical carcinoma, while rare, is frequently hormonally active.