Aims: Acute heart failure (AHF) is the leading cause of hospital admissions among adults ≥65. Loop diuretics are the mainstay of treatment of congestion in AHF. Response to loop diuretics is closely related to morbidity and mortality. In this study, we aimed to investigate (1) the clinical determinants of diuretic efficiency (DE) by using three separate indicators for assessing DE and (2) the prognostic effect of diuretic efficiency in acutely decompensated heart failure patients. Methods: 42 consecutive patients admitted to the hospital for acutely decompensated heart failure were included. Early diuretic response, spot urine sodium excretion (UNa), and hemoconcentration were evaluated individually to predict loop diuretic efficiency. Results: Good early diuretic response (EDR) was associated with higher diastolic blood pressure on admission and eGFR, atrial fibrillation, and bolus dosing of intravenous furosemide. Hypertension and low systolic blood pressure on admission were inversely related to hemoconcentration. Conclusion: Diuretic efficiency is strongly influenced by the heart rhythm, renal function, blood pressure, prevalence of hypertension, and schedule of furosemide administration. We developed a novel, 3-variable index called DES (diuretic efficiency score) that predicts mortality in AHF patients. Future research in larger cohorts is needed to validate DES as a predictor of mortality in heart failure.