Introduction: Loop diuretics are a first-line treatment for patients hospitalized with congestive HF, yet diuretic resistance affects 25-30% of patients and is a predictor of death, readmission, or renal complications. The addition of metolazone can help overcome resistance, however there is a lack of large-scale trials to establish clinical efficacy and safety of dual diuretic therapy. Our study aims to determine the effects of metolazone in combination with a loop diuretic on net urine output, weight change, absolute change in BUN, length of stay, in-hospital death and hospital readmissions. Methods: In the Scripps Health system, 10,713 chronic HF patients experienced a HF admission from June 2019 through June 2021. Of these, 9,755 patients received loop diuretics while hospitalized, and 958 received metolazone with loop diuretics. Patients were propensity-matched 1:1 based on age, sex, left ventricular ejection fraction, tricuspid valve regurgitation, glomerular filtration rate and blood urea nitrogen, leaving 309 matched patients in each group. The primary outcome was net urine output of the hospitalization. Secondary outcomes include net change in weight for the hospitalization, absolute change in BUN, 30-day readmissions, 90-day readmissions, length of stay and in-hospital deaths. Results: During HF admissions, adjunct metolazone with a loop diuretic was associated with a significantly larger net urine output (-5.6 L vs. -2.1 L, p < 0.0001), net weight change (-10.1 lbs vs. -3.9 lbs, p < 0.0001), and absolute change in BUN (38 mg/dL vs. 24.6 mg/dL, p < 0.0001) for the hospitalization compared to patients who adequately responded to loop diuretics in this patient population. Adjunct metolazone showed no difference in in-hospital deaths (8.7% vs. 8.7%, p > 0.9999) or 30-day readmissions (23.3% vs. 17.5%, p = 0.089), however, it was associated with a significantly higher 90-day readmission rate (33.98% vs. 26.21%, p = 0.044) and length of stay (IQR 5-16 days vs. 3-10 days, p < 0.0001). Conclusions: Adjunct metolazone use may be an effective strategy for this patient population with inadequate response to loop diuretics, keeping 30-day readmission rates and in-hospital mortality similar to patients who adequately respond to loop diuretics.