This study investigates the association between maximal functional capacity (peakVO2) and N-terminal pro-brain natriuretic peptide (NT-proBNP) in 133 ambulatory patients with heart failure with preserved ejection fraction (HFpEF), focusing on patients with obesity. Across all participants, NT-proBNP inversely correlated with peakVO2. However, this association varied based on obesity status. In patients without obesity, there was an inverse relationship between NT-proBNP and peakVO2, while no significant correlation was observed in patients with obesity. These findings suggest that in stable ambulatory HFpEF, NT-proBNP did not predict peakVO2 in patients with obesity. •Higher NT-proBNP levels were significantly but marginally associated with lower peakVO2 in symptomatic patients with HFpEF.•The association between NT-proBNP and peakVO2 largely differed across obesity, being non-significant in patients with obesity.•These findings endorse the limitations of natriuretic peptides in assessing patients with HFpEF and obesity.