BACKGROUND A silent corticotroph-cell adenoma (SCCA) is defined as evidence of immunoreactive ACTH in tumor cells of a pituitary adenoma that is not associated with symptoms of Cushing's disease. Most previous reports have discussed the etiology of this unique entity; however, the precise mechanisms of its "silence" are not clear yet. We discuss the pitfalls of endocrinological diagnosis and surgical management of SCCA. METHODS A 63-year-old man, without the symptoms of Cushing's disease, demonstrated elevated serum levels of ACTH, but normal levels of cortisol. ACTH was measured by newly developed immunoradiometric assays (RMA), which are more specific for biologically active ACTH than conventional radioimmunoassay (RIA). The tumor was more than 3 cm in diameter and invaded both the cavernous sinus and the sphenoid sinus. RESULTS Two different types of IRMAs revealed elevated serum ACTH levels. The tumor was removed successfully by staged operations. The high levels of serum ACTH were normalized after the second operation. CONCLUSIONS The cause of the silence of SCCA is not clear yet. The elevated serum ACTH levels in the present case were not attributable to "Big ACTH." We suggest one possibility: the cause of the silence might be a minor variant of ACTH, detected by IRMA, leading to less biological activity of ACTH. This type of adenoma is invasive in nature and can potentially grow large because it is free of endocrinological symptoms. We emphasize the appropriate surgical strategy to normalize the serum levels of ACTH in case of SCCA.