A 33-year old male was diagnosed with Cushing's disease due to a large and invasive ACTH-secreting macroadenoma. After surgical failure ketoconazole therapy was initiated to control cortisol hypersecretion and his symptoms. He was referred to radiotherapy, and fractionated stereotactic radiotherapy in 30 fractions was delivered. After 12 daily fractions of radiotherapy the urinary cortisol release increased abruptly together with clinical deterioration. The daily ketoconazole dose was increased, and 10 days after concluding radiotherapy his urinary cortisol returned to normal values. Hormonal remission was observed less than 1 year following radiotherapy.