A 29-year-old man had a six-month history of fatigue and hypokalemia. Gastrointestinal losses of potassium were not judged significant. The patient denied ingestion of licorice, large quantities of laxatives, or diuretics. Clinical and laboratory findings were consistent with Bartter's syndrome in the adult. Normal blood pressure, hypokalemic alkalosis, and hyperaldosteronism, with insensitivity to the pressor effect of angiotensin infusion, were present. Another major finding in Bartter's syndrome, juxtaglomerular hyperplasia, was not demonstrated because plans for renal biopsy were cancelled when thiazide was detected in the urine, utilizing chemical extraction and spectrophotometry. Surreptitious ingestion of diuretics must be excluded in any adult patient in whom a diagnosis of Bartter's syndrome is considered.