The lack of specific immunohistochemical markers for adrenocortical tumors often causes problems in pathologic diagnosis of adrenal neoplasms. Inhibin is a glycoprotein that is normally expressed in gonadal sex cord stromal elements and tumors and in normal adrenocortical cells. The immunohistochemical expression of inhibin A was studied in a series of 58 adrenal tumors, which included pheochromocytomas, adrenocortical adenomas and carcinomas, and metastatic carcinomas. Twelve renal cell carcinomas were also evaluated. Normal adrenal cortex showed a moderately uniform staining pattern: weak staining in the glomerular layer, moderate in the fascicular layer, and a strong and diffuse reactivity in the reticular zone. All 20 adrenocortical adenomas were strongly and diffusely positive, although those associated with hyperaldosteronism exhibited a patchy staining distribution. Seven of 8 adrenocortical carcinomas were strongly and diffusely positive, whereas 10 of 11 pheochromocytomas were negative. Conversely, inhibin was not detected in any adrenal metastases and renal cell carcinomas. The results suggest that inhibin A may be a good marker for adrenocortical tumors and may be of great interest in the differential diagnosis with pheochromocytomas, metastatic tumors, and renal cell carcinomas that spread into the retroperitoneum.