BACKGROUND: Current guidelines lack instruction on fluid administration for patients with critically ill heart failure (HF) in the intensive care unit. OBJECTIVES: This study aims to compare the risk of mortality and acute kidney injury (AKI) outcomes among unlimited intake, restrictive intake, and negative balance. DESIGN: Retrospective cohort study. METHODS: A total of 3267 patients with HF not receiving renal replacement therapy were included from the Medical Information Mart for Intensive Care-IV (MIMIC-IV) database and were grouped into restrictive fluid intake (the first 24-h fluid intake from 1500 to 2000 mL), negative fluid balance management (the first 24-h net output >1000 mL), and unlimited fluid control groups. The primary endpoint was 28-day mortality, and the secondary endpoints were morbidity of 48-h AKI, 7-day AKI, and AKI progression from 48 h to 7 days. The odds ratio (OR) and the 95% confidence interval (CI) were estimated by multivariable logistic regression, and mediation analyses were performed. RESULTS: Negative fluid balance, not restrictive fluid intake, was correlated to a lower risk of 28-day mortality (OR: 0.750 (0.570-0.987), p = 0.040), 48-h AKI (OR: 0.207 (0.169-0.255), p < 0.001), and 7-day AKI (OR: 0.261 (0.207-0.330), p < 0.001) but was correlated to a higher risk of AKI progression (OR: 2.284 (1.835-2.843), p < 0.001) compared to unlimited fluid control after multivariable adjustment. The 24-h net output mediated AKI incidence and progression. CONCLUSION: Negative fluid balance was related to lower risk of 28-day mortality, 48-h AKI, and 7-day AKI but a higher risk of AKI progression, which requires further clinical trials for validation. The first 24-hour negative fluid balance is related to lower short-term mortality but higher AKI progression in critically ill heart failure: a retrospective cohort studyThe first 24-hour negative fluid balance is correlated to lower 28-day mortality. Acute kidney injury progression should be cautioned in the first 24-hour negative fluid balance. Lower fluid output and more fluid input may prevent acute kidney injury incidence. The first 24-hour net output mainly contributes to the mediation effect.