OBJECTIVE Patients with asymptomatic aortic stenosis (AS) and abnormal haemodynamic responses to exercise testing are at increased risk of cardiac events. This study assesses the Doppler echocardiographic determinants of a positive exercise test in a cohort of asymptomatic patients with AS. RESULTS One hundred and twenty-eight patients with AS underwent quantitative Doppler echocardiographic measurements at rest and during exercise test. Of these patients, 60 had an abnormal response to exercise. Two independent determinants of an abnormal exercise response were selected in multivariate analysis: a larger increase in mean transaortic pressure gradient (P = 0.00014) and a limited contractile reserve-latent left ventricular dysfunction-as indicated by smaller changes in ejection fraction (P = 0.0002). Limiting symptoms were associated with greater increase in mean transaortic pressure gradient, smaller changes in systolic blood pressure and a lower ejection fraction at peak exercise. The increase in pressure gradient was associated with smaller exercise-induced changes in aortic valve area and in ejection fraction and new or worsening mitral regurgitation during exercise. CONCLUSIONS Abnormal responses to exercise in asymptomatic AS patients are mediated by a larger increase in mean transaortic pressure gradient and/or a limited contractile reserve characterized by an inadequate increase in ejection fraction at exercise.