Background: Although half of all patients with heart failure (HF) have a normal or near-normal ejection fraction and their prognosis differs little from that of patients with a reduced ejection fraction, the pathophysiology of HF with preserved ejection fraction (HF-PEF) is still poorly understood, and its management poorly supported by clinical trials. Sodium and fluid restriction is the most common self-care measure prescribed to HF patients for management of congestive episodes. However, its role in the treatment of HF-PEF remains unclear. This trial seeks to compare the effects of a sodium- and fluid-restricted diet versus an unrestricted diet on weight loss, neurohormonal activation, and clinical stability in patients admitted for decompensated HF-PEF. Methods/Design: This is a randomized, parallel trial with blinded outcome assessment. The sample will include adult patients (aged ≥18 years) with a diagnosis of HF-PEF admitted for HF decompensation. The patients will be randomized to receive a diet with sodium and fluid intake restricted to 0.8 g/day and 800 mL/day respectively (intervention group) or an unrestricted diet, with 4 g/day sodium and unlimited fluid intake (control group), and followed for 7 days or until hospital discharge. The primary outcome shall consist of weight loss at 7 days or discharge. The secondary outcome includes assessment of clinical stability, neurohormonal activation, daily perception of thirst and readmission rate at 30 days. Discussion: Assessment of the effects of sodium and fluid restriction on neurohormonal activation and clinical course of HF-PEF can promote a deeper understanding of the pathophysiology and progression of this complex syndrome.