The diagnosis of the ectopic ACTH syndrome often remains difficult. Although bilateral inferior petrosal sinus sampling has recently offered a new approach, it does not help to localize an occult nonpituitary tumor. We report the case of a 45-yr-old woman whose hypercortisolism highly suggested the ectopic ACTH syndrome: elevated urinary free cortisol (3234 nmol/day, normal 28-143) was not suppressed by the high-dose dexamethasone test (2789 nmol/day); increased plasma ACTH (21.8 pmol/L, normal 2-11.4) did not respond to the ovine CRH test (23.8 pmol/L); and pituitary magnetic resonance imaging was negative. The thorax computed tomographic scan showed a questionable 7-mm nodular lesion in the upper part of the left lung. Because a 3-day trial of octreotide administration (200 micrograms sc every 8 h) induced a dramatic clinical and biological response with a drop in urinary free cortisol from 1738 to 441 nmol/day we performed a scintigraphy with [111In]pentetreotide; it revealed a single-well limited area of abnormal uptake at the exact location of the suspected thoracic lesion. This nodule was removed surgically after preparation of the patient by a 1-month treatment with octreotide: the tumor proved to be a typical bronchial carcinoid, containing extremely high concentrations of immunoreactive ACTH (198 pmol/mg wet wt tissue) and POMC messenger RNA by Northern blot. The presence of somatostatin receptors in the tumor was confirmed by in vitro radioautography. After surgery plasma cortisol and ACTH were undetectable. Somatostatin radioanalog scintigraphy should be considered as a new investigative tool in patients with suspected ectopic ACTH syndrome.