A case of Cushing's syndrome in a 36‐year‐old woman, in whom the corticosteroid levels remained elevated in peripheral blood despite high doses of dexamethasone (8 mg/day), is reported. Although, on her skull X‐ray film, the pituitary fossa was seen to be asymmetrically enlarged and had signs of possible erosion and depression of the floor on the left, the question whether this was an anatomical variant was raised. The correct diagnosis was made after measuring ACTH levels in venous blood samples taken from multiple sites, including the left petrosal sinus, in which a significant elevation of ACTH level, compared with the level in peripheral blood, was found.