Background—The urinary composition, including sodium (Na+) and chloride (Cl−) concentrations, might provide useful information in addition to urine output during decongestive treatment in heart failure. Methods and Results—Consecutive patients with heart failure (n=61), ejection fraction ≤45%, worsening symptoms, and scheduled treatment with intravenous loop diuretics were included. Patients received protocol-driven therapy until complete decongestion, assessed clinically and by echocardiography. Three consecutive 24-hour urinary collections were performed. With 2 mg (1–4 mg), 1 mg (0–2 mg), and 1 mg (0–1 mg) bumetanide administered in bolus during consecutive 24-hour intervals, in addition to combinational diuretic therapy in ≈70% and both oral spironolactone and vasodilators in ≈90%, euvolemia was reached, often within 24 hours. Urine output was higher during the first when compared with the second or third 24-hour interval (2700 versus 1550 or 1375 mL, respectively; P<0.001), but th