• A 16-year-old female patient with recurrent Cushing's disease (CD) underwent successful treatment with pituitary irradiation. Within one year after radiotherapy, cortisol levels had returned to normal (but with continued absence of diurnal variation), growth velocity improved, and puberty ensued. Five years after treatment, the patient developed clinical and biochemical evidence of recurrent CD. The high baseline evening corticotropin level (9 pmol/L [40 pg/mL]) was unresponsive (maximum level, 10 pmol/L [46 pg/mL]) to stimulation with ovine corticotropin-releasing hormone (CRF). In patients with CD treated with radiotherapy, the corticotropin response to CRF stimulation may not be reliably compared with that of normal control values. After pituitary adenomectomy, the corticotropin concentration was still unresponsive to CRF. We suggest that the pituitary tumor was secondary to abnormal hypothalamic CRF regulation not corrected by pituitary irradiation; therefore, CD may recur despite pituitary irradiation.