A patient with profound hypokalemia satisfied the criteria for Bartter's syndrome, including hyperreninemia, aldosteronism, normal blood pressure, and hyperplasia of the juxtaglomerular apparatus. Two screening tests of urine and one of plasma for diuretic agents gave negative results. A third urinary sample gave negative results for thiazide but positive for furosemide; the fourth and fifth samples gave negative results for furosemide but positive for thiazide. Urinary prostaglandin excretion was normal. We conclude that this apparent case of Bartter's syndrome was caused by long term surreptitious diuretic ingestion and suggest this may occur more frequently than is generally appreciated.