A 48-year-old man was admitted for treatment of Cushing's syndrome due to right adrenal adenoma, associated with chronic renal failure (CRF) with a blood urea nitrogen level of 64.2 and serum creatinine level of 3.9 mg/dl. After removal of the adrenal adenoma, the CRF deteriorated with progressive symptoms of anorexia, vomiting and hypertension, and the patient was placed on hemodialysis. Prior to adrenalectomy, the 17 OHCS and 17 KGS in the urine were not so high. However, the urinary 17 KS was high with an elevated 11-oxy fraction. In comparison with 2 patients suffering from adrenal Cushing's syndrome with normal renal function, there were no large accumulated quantities of glucuronic conjugated and unconjugated metabolites in the plasma of the CRF Cushing's syndrome, with confirmation ascribable to the radioimmunoassayable cross-reactivity of the cortisol antiserum used in the radioimmunoassay kit. In the Cushing's syndrome with CRF, almost all the cortisol, which was hypersecreted from the adenoma, was presumed to be converted to the 11-oxy fraction of 17 KS, possibly by activation of hepatic enzymes.