IMPORTANCE The benefits of fluid and sodium restriction in patients hospitalized with acute decompensated heart failure (ADHF) are unclear. OBJECTIVE To compare the effects of a fluid-restricted (maximum fluid intake, 800 mL/d) and sodium-restricted (maximum dietary intake, 800 mg/d) diet (intervention group [IG]) vs a diet with no such restrictions (control group [CG]) on weight loss and clinical stability during a 3-day period in patients hospitalized with ADHF. DESIGN Randomized, parallel-group clinical trial with blinded outcome assessments. SETTING Emergency room, wards, and intensive care unit. PARTICIPANTS Adult inpatients with ADHF, systolic dysfunction, and a length of stay of 36 hours or less. INTERVENTION Fluid restriction (maximum fluid intake, 800 mL/d) and additional sodium restriction (maximum dietary intake, 800 mg/d) were carried out until the seventh hospital day or, in patients whose length of stay was less than 7 days, until discharge. The CG received a standard hospital diet, with liberal fluid and sodium intake. MAIN OUTCOMES AND MEASURES Weight loss and clinical stability at 3-day assessment, daily perception of thirst, and readmissions within 30 days. RESULTS Seventy-five patients were enrolled (IG, 38; CG, 37). Most were male; ischemic heart disease was the predominant cause of heart failure (17 patients [23%]), and the mean (SD) left ventricular ejection fraction was 26% (8.7%). The groups were homogeneous in terms of baseline characteristics. Weight loss was similar in both groups (between-group difference in variation of 0.25 kg [95% CI, -1.95 to 2.45]; P = .82) as well as change in clinical congestion score (between-group difference in variation of 0.59 points [95% CI, -2.21 to 1.03]; P = .47) at 3 days. Thirst was significantly worse in the IG (5.1 [2.9]) than the CG (3.44 [2.0]) at the end of the study period (between-group difference, 1.66 points; time × group interaction; P = .01). There were no significant between-group differences in the readmission rate at 30 days (IG, 11 patients [29%]; CG, 7 patients [19%]; P = .41). CONCLUSIONS AND RELEVANCE Aggressive fluid and sodium restriction has no effect on weight loss or clinical stability at 3 days and is associated with a significant increase in perceived thirst. We conclude that sodium and water restriction in patients admitted for ADHF are unnecessary. TRIAL REGISTRATION clinicaltrials.gov Identifier: NCT01133236.