Endogenous Cushing's syndrome is an endocrine disease resulting from chronic exposure to excessive glucocorticoids produced in the adrenal cortex. Although the ultimate outcome remains uncertain, functional and morphological brain changes are not uncommon in patients with this syndrome, and generally persist even after resolution of hypercortisolemia. We present an adolescent patient with Cushing's syndrome who exhibited cognitive impairment with brain atrophy. A 19-year-old Japanese male visited a local hospital following 5 days of behavioral abnormalities, such as money wasting or nighttime wandering. He had hypertension and a 1-year history of a rounded face. Magnetic resonance imaging (MRI) revealed apparently diffuse brain atrophy. Because of high random plasma cortisol levels (28.7 μg/dL) at 10 AM, he was referred to our hospital in August 2011. Endocrinological testing showed adrenocorticotropic hormone-independent hypercortisolemia, and abdominal computed tomography demonstrated a 2.7 cm tumor in the left adrenal gland. The patient underwent left adrenalectomy in September 2011, and the diagnosis of cortisol-secreting adenoma was confirmed histologically. His hypertension and Cushingoid features regressed. Behavioral abnormalities were no longer observed, and he was classified as cured of his cognitive disturbance caused by Cushing's syndrome in February 2012. MRI performed 8 months after surgery revealed reversal of brain atrophy, and his subsequent course has been uneventful. In summary, the young age at onset and the short duration of Cushing's syndrome probably contributed to the rapid recovery of both cognitive dysfunction and brain atrophy in our patient. Cushing's syndrome should be considered as a possible etiological factor in patients with cognitive impairment and brain atrophy that is atypical for their age.