BACKGROUND: Loop diuretics are considered first-line therapy for congestion in children with heart failure, although some patients remain volume overloaded during treatment. We sought to characterize loop diuretic responsiveness (DR) in children hospitalized with acute decompensated failure and to determine whether a decreased response was associated with worse outcomes. METHODS AND RESULTS: DR was calculated for 108 consecutive children <21 years of age who were hospitalized with acute decompensated heart failure. DR was defined as net fluid (mL) output per 1 mg of furosemide equivalents during the first 72 hours of treatment with a loop diuretic. The primary outcome was the composite end point of inpatient death or use of mechanical circulatory support. The median DR was 6.0 mL/mg (interquartile range -2.4 to 15.7 mL/mg). Thirty-two percent of patients remained in a positive fluid balance after 72 hours of treatment with a loop diuretic. Death or use of mechanical circulatory support occurred in 29 patients (27%). Low DR was associated with the composite end point, even after adjusting for net urine output and loop diuretic dose indexed to weight (odds ratio 5.3; P=.003). Patients with low DR also experienced longer length of hospital stay than patients with greater DR (median 33 days vs 11 days; P=.002). CONCLUSION: In children hospitalized with acute decompensated heart failure, early diminished loop DR during decongestion therapy is common and portends a poor prognosis.