Corticotropin-dependent Cushing's syndrome was detected in a 32-year-old male suffering from membranous nephropathy and chronic renal failure. Cortisol dynamics revealed high basal cortisol, loss of circadian rhythm, and nonsuppressibility with low-dose dexamethasone. However, the latter was suppressible with high-dose dexamethasone. Treatment with ketoconazole led to a remarkable response both clinically and biochemically. The occurrence of Cushing's syndrome in a patient with chronic renal failure is extremely rare and poses significant diagnostic and therapeutic problems.