This study describes a case of pituitary-dependent Cushing's syndrome where standard biochemical and radiological techniques were discordant in localizing the origin of the autonomous adrenocorticotropic hormone (ACTH) hypersecretion in the pituitary. Hormonal evaluation suggested a pituitary genesis for the disease, but both sellar computed tomography and cranial magnetic resonance (MR) were unable to give clear-cut evidence for a pituitary neoplasm. Simultaneous and bilateral inferior petrosal sinus sampling (SBIPS) correctly identified the left side of the pituitary gland as the source of autonomous ACTH production. One year later, the shaded signs of a pituitary lesion in the left side of the gland were seen with MR imaging, and a 0.5-cm in height adenoma was surgically removed. At the 2-year follow-up the patient's symptoms had completely disappeared, and her menses were restored. In this case, SBIPS correctly diagnosed the presence of an ACTH-secreting pituitary microadenoma one year before shaded signs of the pituitary lesion appeared with MR imaging. This is a clear-cut demonstration of the accuracy of the SBIPS technique in localizing small pituitary lesions.