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Background and aims Obesity is associated with lower N-terminal pro-B-type natriuretic peptide (NT-proBNP) concentration, possibly because obese patients become symptomatic with less severe left ventricular dysfunction. We explored the relation among body mass index (BMI), left ventricular ejection fraction (LVEF), symptoms, NT-proBNP and mortality in ambulatory patients investigated for suspected heart failure (HF).Methods Between 2001 and 2023, patients referred to a community HF clinic had age, sex, BMI, plasma NT-proBNP, serum creatinine, LVEF, New York Heart Association (NYHA) class, cardiac rhythm and systolic blood pressure recorded. Patients were categorised as ‘lean’ BMI <25 kg/m 2, ‘overweight’ BMI 25–29.9 kg/m2 or ‘obese’ BMI ≥30 kg/m2.Results Among 2866 patients (mean age 71±11 years; 63% men), NT-proBNP concentrations were inversely related to BMI after adjustment for age, sex, LVEF, renal function, rhythm, symptoms and blood pressure (p<0.001). Each 5 kg/m² increase in BMI was associated with a 13% (95% CI 10% to 17%) lower NT-proBNP. Within each NYHA class and across most LVEF values, patients with higher BMI had lower NT-proBNP concentrations. However, within NT-proBNP strata, BMI was not associated with mortality. Patients with NT-proBNP <125 ng/L had <1% 1-year mortality, irrespective of BMI.Conclusions In ambulatory patients with suspected HF, higher BMI is associated with lower NT-proBNP concentrations but does not attenuate the prognostic value of NT-proBNP. Lowering guideline-recommended NT-proBNP thresholds for obese patients is unlikely to identify individuals at higher mortality risk.