Biologically inactive ACTH-producing pituitary adenoma is known as clinically silent corticotroph adenoma. To search for the mechanism causing clinically silent corticotroph adenoma, we immunohistochemically examined the expression of prohormone convertase 1/3 (PC1/3) in this type of adenoma and compared our results with those obtained for Cushing's disease. All of the Cushing's disease specimens exhibited strongly positive PC1/3 exhibition. On the contrary, the expression of PC1/3 was very weak in the clinically silent corticotroph adenoma specimens. The absence of PC1/3 in clinically silent corticotroph adenoma indicates that silent corticotroph adenomas arise in a different cell type sharing the prohormone pro-opiomelanocortin (POMC), but processing it differently, accounting for the lack of clinical symptoms due to ACTH excess.