BackgroundThe aim of this study is to investigate origin, gross features, microscopic features, immunohistochemical properties, and differential diagnosis of adrenal cortical adenoma (ACA) in patients ≥20 years old.MethodsThe clinicopathological features of 116 cases of ACA and the immunohistochemical features of 50 cases of ACA were evaluated, and the relevant literature was reviewed.ResultsIn our cohort, 76.72% (89/116) of the cases were functional, and 27 cases had non-functional, benign adrenal adenomas. ACA presented as an island tumor with an envelope, and the mean tumor size was 3.6 cm (range 1–5 cm), with a mean tumor weight of 9.28 g (range 5–113 g). The shape of the tumor cells was consistent, and mitosis was rarely observed. Forty of the 46 patients with cortisol-secreting ACA had tumors containing granule cells. Primary aldosteronism was observed in 43 cases. Thirty-eight cases had endoscopically visible tumors, with clear cells and lipid-rich cytoplasm arranged in irregular patches or strips. Cortisol-producing ACAs were associated with atrophy of the non-tumorous cortex. Adrenocortical adenomas displayed positive immunohistochemical staining for MELAN-A, Syn (46 of 50 cases of ACA), NSE (44 of 50 cases of ACA), Vim (42 of 50 cases of ACA) and Ki-67 <5% (24 of 50 cases of ACA; the remaining 26 cases were negative for Ki-67).ConclusionPrediction of endocrine syndrome in functional ACA was possible based on its structure and morphologic features, which could prevent an unanticipated postoperative crisis. However, a clinical study is needed to validate these findings.