Doppler echocardiography has been shown to be an accurate method of assessing left ventricular outflow obstruction in hypertrophic cardiomyopathy (HCM). One of the characteristics of this pressure gradient is its variability and, therefore, we measured this parameter during dynamic exercise testing in 33 patients. The results were compared with those recorded during isoproterenol infusion, the reference stress test for patients with HCM. Submaximal exercise in the recumbent position is usually well tolerated and resulted in a 43% increase in heart rate and a 47% increase in pressure gradient. There was a significant correlation between resting and exercise outflow obstruction (r = 0.90; p = 0.001). Moreover, exercise echo revealed obstruction in 26% of patients without resting pressure gradients (latent obstruction). The interpretation of results obtained with isoproterenol infusion is more difficult: this test resulted in an important increase in the left ventricular pressure gradient (231%) and "revealed" obstruction in 84% of cases. Therefore, we believe that exercise is more physiological and better tolerated than isoproterenol stress infusion and should be adopted as the investigation of choice in HCM even without obstruction at rest. If it is not possible to perform the exercise or no outflow tract obstruction can be demonstrated, an isoproterenol infusion may be used but this is not always well tolerated and the results should be interpreted with caution.