Abstract Background Guidelines recognize the role of exercise stress testing in evaluating functional capacity in patients with asymptomatic severe aortic stenosis (AS). Exercise stress echocardiography (ESE) allows the detection of pulmonary congestion with B-lines by lung ultrasound (LUS). Aim To identify the prevalence, determinants, and functional correlates of B-lines during ESE in asymptomatic severe AS. Methods We enrolled 35 asymptomatic patients (age= 69±16 years, 69% male) with severe AS, aortic valve area (AVA) <0.6 cm2/m2, mean aortic gradient (MAG) ≥ 40 mmHg, left ventricular ejection fraction ≥55%. All patients were referred for ESE (treadmill in 1, bicycle in 34 patients) in 7 experienced centers from 4 countries between 2021-2022. ESE assessment at rest and peak stress included global longitudinal strain (GLS), left atrial volume index (LAVI) and cardiac output (CO). B-lines were assessed by simplified LUS from 4 lung zones across the third intercostal space (2 on each side). Heart rate reserve (HRR) was measured as the peak/rest heart rate. CO reserve was calculated as the difference between peak stress CO and rest CO. Results B-lines (≥2) were present in 2 patients at rest and in 15 patients during ESE (6% vs 43%, p<0.001). When compared to patients without B-lines during ESE (Group 1, n=20), patients with B-lines during ESE (Group 2, n=15) were older (Group 1= 64±18 vs Group 2=75±8 years, p=0.030) had larger LAVI, higher logNT-pro-BNP and a trend to higher systolic pulmonary artery pressures (SPAP), see Table. The 2 groups showed similar values of resting AVA (Group 1= 0.52±0.11 vs Group 2=0.47±0.15 cm2/m2, p=0.311), stress AVA (Group 1= 0.45±0.09 vs Group 2=0.49±0.13 cm2/m2, p=0.663), resting MAG (Group 1= 45±15 vs Group 2=46±10 mm Hg, p=0.491), and stress MAG (Group 1= 59±19 vs Group 2=58±11 mm Hg, p=0.665). Patients with B-lines during ESE showed lower HRR and CO reserve, with a trend towards lower peak GLS, see Table. Conclusions Exercise-induced pulmonary congestion as evidenced by development of sonographic B- lines on LUS were present commonly (43%) among patients with "asymptomatic" severe AS undergoing ESE. Patients with exercise-induced B-lines have larger resting LAVI, higher NT-pro-BNP levels, lower CO reserve, and blunted HRR. Exercise-induced B-lines might provide an early sign of cardiac decompensation in "asymptomatic" patients with severe AS, which is an important factor to consider in the decision to intervene.TableFigure