A 12 yr old girl presented with typical features of Cushing syndrome. Plasma F (17.8 μg/dl) showed a normal diurnal rhythm and dexamethasone (DX)-suppressibility. Urinary 17 KS (11.1 mg/24 h) and 170H-C (20.1 mg/24 h) were increased but DX-suppressible, THE and THF showed a partial response to DX only. Free urinary F was 160 μg/m2/24 h (normal : < 50) and DX-suppressible. Plasma ACTH was 160 pg/ml (normal : < 100). Treatment with aminoglutethimide (4 × 500 mg/day) for 8 months had a transitory effect only and cyproheptadine (16 mg/day) for 1 yr was unsuccessful. Plasma ACTH increased to 340 pg/ml. Sellar enlargement became evident. At transsphenoidal pituitary exploration a cystic microadenoma was resected. After surgery and pituitary irradiation transient ACTH deficiency was observed. Menstruation started 3 months post-operatively. T4 and T3 remained normal. The blunted TSH response to TRH was normalized. Responses of LH and FSH to LHRH, PRL to TRH and GH to arginine were normal.