Abstract Aims Studies have demonstrated the reliability of B‐lines evaluated by lung ultrasonography to identify pulmonary congestion, but information is lacking about its utility as a prognostic marker of heart failure (HF). We sought to assess the prognostic midterm value of B‐lines in ambulatory patients presenting with dyspnoea, as an additive tool for patient management and to avoid acute HF exacerbations. Methods and results A total of 93 patients presenting with dyspnoea (New York Heart Association ≥2) were prospectively recruited in an outpatient clinic, and underwent clinical and echocardiographic evaluation, as well as B‐line evaluation with lung ultrasonography in eight zones. Primary endpoint was HF hospitalization at 1 year. A total of 88 patients were included, age 72.3 ± 9.6, with left ventricular ejection 47.7 ± 28.6%; E/e' 16.9 ± 10.9, left atrial volume 51.9 ± 22.5 mL/m2; peak tricuspid regurgitation velocity 2.6 ± 0.5 m/s, average B‐line count 7.7 ± 10. 8 (9%) patients were hospitalized for HF, seven of which had ≥6 B‐lines. B‐line cut‐off ≥6 (specificity = 66.2%; sensitivity = 87.5%) was predictive for HF hospitalization, with an odds ratio at 13.7 for HF hospitalization at 1 year [IC95% (1.6–117.4), P = 0.017]. Conclusions Ambulatory patients with ≥6 B‐lines have a higher risk of HF hospitalization at 1 year. This study highlights the prognostic value of B‐lines in evaluating HF risk in dyspnoeic patients.